Stylized Water 2 - Sample underwater mask

Shader "Stylized Water 2/Underwater Masking test"
{
    Properties
    {
        _Color("Color", Color) = (1,1,1, 1)
    }

    SubShader
    {
        Tags { "RenderType" = "Transparent" "Queue" = "Transparent" "RenderPipeline" = "UniversalPipeline" }
        Blend SrcAlpha OneMinusSrcAlpha
        ZWrite Off

        Pass
        {
            Tags { "LightMode" = "UniversalForward" }

            HLSLPROGRAM
            #pragma target 3.0
            #pragma vertex vert
            #pragma fragment frag

            //Using a shader variant is optional, as the code is still conditionally executed
            #pragma multi_compile_fragment _ UNDERWATER_ENABLED

            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"

            float4 _Color;

            //Declare these properties
            bool _FullySubmerged, _UnderwaterRenderingEnabled;
            TEXTURE2D_X(_UnderwaterMask);
            //Note: you can use any sampler, as long as it doesn't use point-filtering
            SAMPLER(sampler_UnderwaterMask);

            struct appdata
            {
                float4 vertex : POSITION;
            };

            struct v2f
            {
                float4 positionCS     : SV_POSITION;
            };

            v2f vert(appdata v)
            {
                v2f o;

                o.positionCS = TransformObjectToHClip(v.vertex.xyz);

                return o;
            }

            half4 frag(v2f input) : SV_Target
            {
                float3 color = _Color.rgb;
                float alpha = _Color.a;
                float2 uv = GetNormalizedScreenSpaceUV(input.positionCS);

                #if UNDERWATER_ENABLED
                //Boolean toggled if the current rendering camera is valid and touching the water line
                if(_UnderwaterRenderingEnabled)
                {
                    //This value will be true if the top of the screen is completely below the water level
                    //If so the pixels in this function should be completely discarded because the underwater fog consumes the visible area
                    if(_FullySubmerged)
                    {
                        alpha = 0;
                    }
                    else
                    {
                        //Screen-space texture. White for parts that are underwater, anything else is black.
                        half underwaterMask = SAMPLE_TEXTURE2D_X(_UnderwaterMask, sampler_UnderwaterMask, uv).r;

                        //Will make the parts that are BELOW the water completely transparent
                        alpha *= 1-underwaterMask;
                    }
                }
                #endif

                return float4(color.rgb, alpha);

            }
            ENDHLSL
        }
    }
}