F# Translation for Miguel. I have no idea if this works, and it is pretty much a verbatim translation, only some F# flare added. I didn't dare to do too much refactoring without having a way to test properly. Original: https://github.com/xamarin/monotouch

FsVersion.fs

namespace GLCameraRipple
  open System
  open System.Drawing
  open System.Runtime.InteropServices
  //open MonoTouch.CoreFoundation
  open Microsoft.FSharp.NativeInterop

  type RippleModel(screenSize : Size, meshFactor: int, touchRadius: int, textureSize: Size) =
    do Console.WriteLine ("New RippleModel");
    let poolWidth  = screenSize.Width  / meshFactor
    let poolHeight = screenSize.Height / meshFactor

    let texCoordFactorS, texCoordOffsetS,
        texCoordFactorT, texCoordOffsetT =
           if (float screenSize.Height) / (float screenSize.Width) < (float textureSize.Width) / (float textureSize.Height) then
              let t = (textureSize.Height * screenSize.Height |> float) / (screenSize.Width * textureSize.Width |> float)
              t  , (1.0 - t) / 2.0,
              1.0, 0.0
           else
              let t = (screenSize.Width * textureSize.Width |> float) / (textureSize.Height * screenSize.Height |> float)
              1.0, 0.0,
              t  , (1.0 - t) / 2.0

    let rippleCoeff = Array2D.init (touchRadius * 2 + 1) (touchRadius * 2 + 1) (fun x y ->
      let distance = (x - touchRadius) * (x - touchRadius) + (y - touchRadius) * (y - touchRadius) |> float |> sqrt
      if distance < (float touchRadius) then
         let factor = distance / (float touchRadius)
         ((factor * Math.PI |> cos) + 1.0) * 256.0
      else 0.0
      )

    // +2 for padding the border, mutable so we avoid array copy
    let mutable rippleSource = Array2D.create (poolWidth + 2) (poolHeight + 2) 0.0
    let mutable rippleDest   = Array2D.create (poolWidth + 2) (poolHeight + 2) 0.0

    let poolSize2 = poolWidth * poolHeight * 2
    let rippleVertices = Marshal.AllocHGlobal(poolSize2 * sizeof<float>);
    let rippleTexCoords = Marshal.AllocHGlobal(poolSize2 * sizeof<float>);
    let rippleIndicies = Marshal.AllocHGlobal((poolHeight - 1) * (poolWidth * 2 + 2) * sizeof<uint16>);

    // Use nativeptr internally, expose nativeint
    let verts     = NativePtr.ofNativeInt<float> rippleVertices
    let texcoords = NativePtr.ofNativeInt<float> rippleTexCoords
    let indices   = NativePtr.ofNativeInt<uint16> rippleIndicies
    
    do // InitMesh()
      for i in 0 .. poolHeight - 2 do
        for j in 0 .. poolWidth - 1 do
          // This is hideous, but can probably be improved with some inlined helper
          NativePtr.set verts ((i * poolWidth + j) * 2 + 0) (-1.0 + (float j) * (2.0 / ((float poolWidth)  - 1.0)))
          NativePtr.set verts ((i * poolWidth + j) * 2 + 1) ( 1.0 - (float i) * (2.0 / ((float poolHeight) - 1.0)))

          NativePtr.set texcoords ((i * poolWidth + j) * 2 + 0) ( (float i) / ((float poolHeight) - 1.0) * texCoordFactorS + texCoordOffsetS)
          NativePtr.set texcoords ((i * poolWidth + j) * 2 + 1) ( (float i) / ((float poolHeight) - 1.0) * texCoordFactorS + texCoordOffsetS)
      let index = ref 0
      let write (v: int) = NativePtr.set indices !index (uint16 v)
                           incr index
      for i in 0 .. poolHeight - 1 do
        for j in 0 .. poolWidth - 1 do
          if i % 2 = 0 then
            if j = 0 then
              (i    ) * poolWidth + j |> write
            (i    ) * poolWidth + j |> write
            (i + 1) * poolWidth + j |> write
            if j = (poolWidth - 1) then
              (i + 1) * poolWidth + j |> write
          else
            if j = 0 then
              (i + 1) * poolWidth + j |> write
            (i + 1) * poolWidth + j |> write
            (i    ) * poolWidth + j |> write
            if j = (poolWidth - 1) then
              (i    ) * poolWidth + j |> write
    
    let simulate () = 
      for y in 0 .. poolHeight - 1 do
        for x in 0 .. poolWidth - 1 do
          // water update
          let a, b, c, d = 
             rippleSource.[x + 1, y    ],
             rippleSource.[x + 1, y + 2],
             rippleSource.[x    , y + 1],
             rippleSource.[x + 2, y + 1]
          let result = (a + b + c + d) / 2.0 - rippleDest.[x + 1, y + 1]
          let diminished = result - result / 32.0
          rippleDest.[x + 1, y + 1] <- diminished
      // texcoords
      for y in 0 .. poolHeight - 1 do
        for x in 0 .. poolWidth - 1 do
          let a, b, c, d = 
             rippleDest.[x + 1, y    ],
             rippleDest.[x + 1, y + 2],
             rippleDest.[x    , y + 1],
             rippleDest.[x + 2, y + 1]
          let inline clamp min max v = if v < min then min elif v > max then max else v
          let s_offset = (b - a) / 2048.0 |> clamp -0.5 0.5
          let t_offset = (c - d) / 2048.0 |> clamp -0.5 0.5
          
          let s_tc = (float y) / (poolHeight - 1 |> float) * texCoordFactorS + texCoordOffsetS
          let t_tc = (float x) / (poolWidth  - 1 |> float) * texCoordFactorT + texCoordOffsetT
          
          NativePtr.set texcoords ((y * poolWidth + x) * 2 + 0) (s_tc + s_offset)
          NativePtr.set texcoords ((y * poolWidth + x) * 2 + 1) (t_tc + t_offset)
      let temp = rippleSource
      rippleSource <- rippleDest
      rippleDest <- temp

    let addRipple (l: PointF) = 
       let xIndex = (int (l.X / (float32 screenSize.Width ))) * poolWidth
       let yIndex = (int (l.Y / (float32 screenSize.Height))) * poolHeight

       for y in yIndex - touchRadius .. yIndex + touchRadius do
         for x in xIndex - touchRadius .. xIndex + touchRadius do
           if x >= 0 && x < poolWidth && y >= 0 && y < poolHeight then
             rippleSource.[x + 1, y + 1] = rippleSource.[x + 1, y + 1] + rippleCoeff.[y - (yIndex - touchRadius), x - (xIndex - touchRadius)] |> ignore

    member this.Vertices   with get () = NativePtr.toNativeInt verts
    member this.TexCoords  with get () = NativePtr.toNativeInt texcoords
    member this.Indices    with get () = NativePtr.toNativeInt indices
    member this.VertexSize with get () = poolWidth * poolHeight * 2 * sizeof<float>
    member this.IndexSize  with get () = (poolHeight - 1) * (poolWidth * 2 + 2) * sizeof<float>
    member this.IndexCount with get () = this.IndexSize / sizeof<uint16>
    member this.RunSimulation() = simulate ()
    member this.InitiateRippleAtLocation(location) = addRipple location

Original.cs

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using MonoTouch.CoreFoundation;

namespace GLCameraRipple
{
   public class RippleModel
   {
      Size screenSize;
      int poolHeight, poolWidth;
      int touchRadius, meshFactor;

      float texCoordFactorS;
      float texCoordOffsetS;
      float texCoordFactorT;
      float texCoordOffsetT;

      // ripple coefficients
      float[,] rippleCoeff;

      // ripple simulation buffers
      float[,] rippleSource;
      float[,] rippleDest;

      // data passed to GL
      unsafe float* rippleVertices;
      unsafe float* rippleTexCoords;
      unsafe ushort* rippleIndicies;

      public RippleModel(Size screenSize, int meshFactor, int touchRadius, Size textureSize)
      {
         Console.WriteLine("New RippleModel");
         this.screenSize = screenSize;
         this.meshFactor = meshFactor;
         this.touchRadius = touchRadius;
         poolWidth = screenSize.Width / meshFactor;
         poolHeight = screenSize.Height / meshFactor;

         if ((float)screenSize.Height / screenSize.Width < (float)textureSize.Width / textureSize.Height)
         {
            texCoordFactorS = (float)(textureSize.Height * screenSize.Height) / (screenSize.Width * textureSize.Width);
            texCoordOffsetS = (1 - texCoordFactorS) / 2f;

            texCoordFactorT = 1;
            texCoordOffsetT = 0;
         }
         else
         {
            texCoordFactorS = 1;
            texCoordOffsetS = 0;

            texCoordFactorT = (float)(screenSize.Width * textureSize.Width) / (textureSize.Height * screenSize.Height);
            texCoordOffsetT = (1 - texCoordFactorT) / 2f;
         }

         rippleCoeff = new float[touchRadius * 2 + 1, touchRadius * 2 + 1];

         // +2 for padding the border
         rippleSource = new float[poolWidth + 2, poolHeight + 2];
         rippleDest = new float[poolWidth + 2, poolHeight + 2];

         unsafe
         {
            int poolsize2 = poolWidth * poolHeight * 2;
            rippleVertices = (float*)Marshal.AllocHGlobal(poolsize2 * sizeof(float));
            rippleTexCoords = (float*)Marshal.AllocHGlobal(poolsize2 * sizeof(float));
            rippleIndicies = (ushort*)Marshal.AllocHGlobal((poolHeight - 1) * (poolWidth * 2 + 2) * sizeof(ushort));
         }
         
         InitRippleCoef();
         InitMesh();
      }

      void InitRippleCoef()
      {
         for (int y = 0; y <= 2 * touchRadius; y++)
         {
            for (int x = 0; x <= 2 * touchRadius; x++)
            {
               float distance = (float)Math.Sqrt((x - touchRadius) * (x - touchRadius) + (y - touchRadius) * (y - touchRadius));

               if (distance <= touchRadius)
               {
                  float factor = (distance / touchRadius);

                  // goes from -512 -> 0
                  rippleCoeff[x, y] = -((float)Math.Cos(factor * Math.PI) + 1f) * 256f;
               }
               else
                  rippleCoeff[x, y] = 0;
            }
         }
      }

      unsafe void InitMesh()
      {
         for (int i = 0; i < poolHeight; i++)
         {
            for (int j = 0; j < poolWidth; j++)
            {
               rippleVertices[(i * poolWidth + j) * 2 + 0] = -1f + j * (2f / (poolWidth - 1));
               rippleVertices[(i * poolWidth + j) * 2 + 1] = 1f - i * (2f / (poolHeight - 1));

               rippleTexCoords[(i * poolWidth + j) * 2 + 0] = (float)i / (poolHeight - 1) * texCoordFactorS + texCoordOffsetS;
               rippleTexCoords[(i * poolWidth + j) * 2 + 1] = (1f - (float)j / (poolWidth - 1)) * texCoordFactorT + texCoordFactorT;
            }
         }

         uint index = 0;
         for (int i = 0; i < poolHeight - 1; i++)
         {
            for (int j = 0; j < poolWidth; j++)
            {
               if (i % 2 == 0)
               {
                  // emit extra index to create degenerate triangle
                  if (j == 0)
                  {
                     rippleIndicies[index] = (ushort)(i * poolWidth + j);
                     index++;
                  }

                  rippleIndicies[index] = (ushort)(i * poolWidth + j);
                  index++;
                  rippleIndicies[index] = (ushort)((i + 1) * poolWidth + j);
                  index++;

                  // emit extra index to create degenerate triangle
                  if (j == (poolWidth - 1))
                  {
                     rippleIndicies[index] = (ushort)((i + 1) * poolWidth + j);
                     index++;
                  }
               }
               else
               {
                  // emit extra index to create degenerate triangle
                  if (j == 0)
                  {
                     rippleIndicies[index] = (ushort)((i + 1) * poolWidth + j);
                     index++;
                  }

                  rippleIndicies[index] = (ushort)((i + 1) * poolWidth + j);
                  index++;
                  rippleIndicies[index] = (ushort)(i * poolWidth + j);
                  index++;

                  // emit extra index to create degenerate triangle
                  if (j == (poolWidth - 1))
                  {
                     rippleIndicies[index] = (ushort)(i * poolWidth + j);
                     index++;
                  }
               }
            }
         }
      }

      public IntPtr Vertices
      {
         get { unsafe { return (IntPtr)rippleVertices; } }
      }

      public IntPtr TexCoords
      {
         get { unsafe { return (IntPtr)rippleTexCoords; } }
      }

      public IntPtr Indices
      {
         get { unsafe { return (IntPtr)rippleIndicies; } }
      }

      public int VertexSize
      {
         get { return poolWidth * poolHeight * 2 * sizeof(float); }
      }

      public int IndexSize
      {
         get { return (poolHeight - 1) * (poolWidth * 2 + 2) * sizeof(ushort); }
      }

      public int IndexCount
      {
         get { return IndexSize / sizeof(ushort); }
      }

      public unsafe void RunSimulation()
      {
         for (int y = 0; y < poolHeight; y++)
         {
            for (int x = 0; x < poolWidth; x++)
            {
               // * - denotes current pixel
               //
               //       a 
               //     c * d
               //       b 

               // +1 to both x/y values because the border is padded
               float a = rippleSource[x + 1, y];
               float b = rippleSource[x + 1, y + 2];
               float c = rippleSource[x, y + 1];
               float d = rippleSource[x + 2, y + 1];

               float result = (a + b + c + d) / 2f - rippleDest[x + 1, y + 1];

               result -= result / 32f;

               rippleDest[x + 1, y + 1] = result;
            }
         }

         for (int y = 0; y < poolHeight; y++)
         {
            for (int x = 0; x < poolWidth; x++)
            {
               // * - denotes current pixel
               //
               //       a
               //     c * d
               //       b

               // +1 to both x/y values because the border is padded
               float a = rippleDest[x + 1, y];
               float b = rippleDest[x + 1, y + 2];
               float c = rippleDest[x, y + 1];
               float d = rippleDest[x + 2, y + 1];

               float s_offset = ((b - a) / 2048f);
               float t_offset = ((c - d) / 2048f);

               // clamp
               s_offset = (s_offset < -0.5f) ? -0.5f : s_offset;
               t_offset = (t_offset < -0.5f) ? -0.5f : t_offset;
               s_offset = (s_offset > 0.5f) ? 0.5f : s_offset;
               t_offset = (t_offset > 0.5f) ? 0.5f : t_offset;

               float s_tc = (float)y / (poolHeight - 1) * texCoordFactorS + texCoordOffsetS;
               float t_tc = (1f - (float)x / (poolWidth - 1)) * texCoordFactorT + texCoordOffsetT;

               rippleTexCoords[(y * poolWidth + x) * 2 + 0] = s_tc + s_offset;
               rippleTexCoords[(y * poolWidth + x) * 2 + 1] = t_tc + t_offset;
            }
         }

         var tmp = rippleDest;
         rippleSource = rippleDest;
         rippleSource = tmp;
      }

      public void InitiateRippleAtLocation(PointF location)
      {
         int xIndex = (int)((location.X / screenSize.Width) * poolWidth);
         int yIndex = (int)((location.Y / screenSize.Height) * poolHeight);

         for (int y = (int)yIndex - (int)touchRadius; y <= (int)yIndex + (int)touchRadius; y++)
            for (int x = (int)xIndex - (int)touchRadius; x <= (int)xIndex + (int)touchRadius; x++)
            {
               if (x >= 0 && x < poolWidth && y >= 0 && y < poolHeight)
                  // +1 to both x/y values because the border is padded
                  rippleSource[x + 1, y + 1] += rippleCoeff[(y - (yIndex - touchRadius)), x - (xIndex - touchRadius)];
            }
      }
   }
}

@cocodrino - Yes, that is a nice feature of F#. I wasn't even trying to compress loc.

Is it correct to remove the 2nd loop from the simulate() method? after the 1st one rippleDest is changed, so the values are different.
0 a
1 c * d
2 b
For each iteration we need to know the values for at least 3 rows, but the actual ones are there only at the 3nd iteration, not the 1st one
To remove the loop something like this is needed

76 for y in 0 .. poolHeight + 1 do
77     for x in 0 .. poolWidth - 1 do
78         if j < poolHeight then
79             // ...
87    if j >= 2 then 
88      let a, b, c, d =
89        rippleDest.[x + 1, y - 2],
90        rippleDest.[x + 1, y],
91        rippleDest.[x , y - 1],
92        rippleDest.[x + 2, y - 1]```

It reduces the array read operations (-1 for each iteration), but adds 2 checks, looks like doesn't worse it
0    a1
1 c1 * d1
2    b1 (=a2)
3 c2 * d2
4    b2 ...

@luajalla No, it is an error to remove the second loop. I remember identifying that problem, but must have then forgotten about it.
It shows that I never actually ran this code. I just typed it out to see what this kind of code (gfx, sim) would look like in F#. :)
I'm slightly ashamed to admit I didn't bother with installing MonoTouch. Obviously I should have. :)