Simple shader that imitates a liquid moving inside a vessel.

!Liquid Volume Shader.md

LiquidControl.cs

// Created by @cortvi
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// This script lets you easily control the amount of fluid,
// but you have to manually set up the height values!
[ExecuteInEditMode]
public class LiquidControl : MonoBehaviour
{
	[Range(0f,1f)]
	public float fillLevel;				// How full it is
	public double fullHeight;            		// Height at which vessel is full
	public double emptyHeight;			// Height at which vessel is empty

	int _Level;
	Material mat;
	float currentLevel;

	private void Update()
	{
		if (fillLevel == currentLevel) return;
		
		var lerp = Mathf.Lerp (emptyHeight, fullHeight, fillLevel);
		mat.SetFloat (_Level, lerp);
		// Cache the value
		currentLevel = fillLevel;
	}

	private void OnEnable () 
	{
		_Level = Shader.PropertyToID ("_Level");
		
		#if UNITY_EDITOR
		if (UnityEditor.EditorApplication.isPlaying)
			mat = GetComponent<Renderer> ().material;
		else
			mat = GetComponent<Renderer> ().sharedMaterial;
		#else
		mat = GetComponent<Renderer> ().material;
		#endif
	}
}

LiquidVolume.shader

/// Created by @cortvi
Shader "Custom/Simple liquid volume"
{
	Properties
	{
		_Color ("Liquid color", Color) = (1,1,1,1)
		// For this shader to work properly, the mesh should
		// have its pivot point on the geometric center of the object.
		_Level ("Fill height (object space)", Float) = 0
	}

	SubShader
	{
		Tags
		{
			// You may want to modify the queue
			// for the object to correctly draw behind other
			// transparent materials, such as a glass vessel
			"Queue"="Transparent"
		}
		Cull Off

		CGPROGRAM
		#pragma surface surf Standard fullforwardshadows vertex:vert
		struct Input
		{
			float3 worldPos;
		};

		fixed4 _Color;
		float _Level;

		void vert (inout appdata_full v, out Input o) 
		{
            		UNITY_INITIALIZE_OUTPUT (Input, o);

			// Calculate view direction in object-space
			float3 viewDir = mul(unity_WorldToObject, float4(_WorldSpaceCameraPos.xyz, 1)).xyz - v.vertex.xyz;
			// Replace the face normal by the view direction
			// this way you cannot see that the object is hollow
			v.normal = normalize(viewDir);
		}

		void surf (Input IN, inout SurfaceOutputStandard s)
		{
			// Get the object Y by its mdoel matrix
			float objectPosY = unity_ObjectToWorld._m13;
			// Calculate the highest point in world-space of the liquid
			float3 targetDistance = objectPosY + _Level;
			// Discard pixels that are over liquid's highest point
			float value = targetDistance - IN.worldPos.y;
			clip(value);

			// Just pass the liquid color.
			// You could actually do some texture work here!
			s.Albedo = _Color;
		}
		ENDCG
	}
	FallBack "Diffuse"
}