What is WaterTextureDepth(float3 posWS) doing in BoatAttack water system?

News & General Discussion
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float WaterTextureDepth(float3 posWS)
{
    return (1 - SAMPLE_TEXTURE2D_LOD(_WaterDepthMap, sampler_WaterDepthMap_linear_clamp, posWS.xz * 0.002 + 0.5, 1).r) * (_MaxDepth + _VeraslWater_DepthCamParams.x) - _VeraslWater_DepthCamParams.x;
}

float3 WaterDepth(float3 posWS, half4 additionalData, half2 screenUVs)// x = seafloor depth, y = water depth
{
	float3 outDepth = 0;
	outDepth.xz = AdjustedDepth(screenUVs, additionalData);
	float wd = WaterTextureDepth(posWS);
	outDepth.y = wd + posWS.y;
	return outDepth;
}

From usage semantics I can guess that WaterTextureDepth return depth by subtract water surface height from terrain height.
But I still can’t get how it is calculated.