How to get caustics only show underwater?

Try using an orthogonal projector instead of directional light. With some additive shader it will create light like effect, however it with lack of shadows. Or at least i don't know about projector having the shadowmap feature supported by the unity, however it could be scripted. So just create an ortho projector facing down and spreading over the water surface. The default unity additive projector shader doesn't clip correctly what's behind, so here's what I am using:

 Shader "Projector/Additive"
 {
     Properties
     {
         _Decal ("Decal", 2D) = "white" { TexGen ObjectLinear   }
         _Color ("Color", Color) = (0.5, 0.5, 0.5, 1)
     }
     
     Subshader
     {
         Pass
         {
             ZWrite On
             Fog { Color (1, 1, 1) }
             Offset -1, -1
             Blend One One
             Cull Back
              
             Tags { "RenderType"="Geometry-10" }
             
             CGPROGRAM
             #include "UnityCG.cginc"
             #pragma vertex vert
             #pragma fragment frag
     
             sampler2D _Decal;
             float4x4 _Projector;
             float4x4 _ProjectorClip;
             float4 _Color;
     
             struct vin
             {
                 float4  pos     :     POSITION;
                 float4  uv        :     TEXCOORD0;
             };
             
             struct v2f
             {
                 float4 vertex    : SV_POSITION;
                 float4 uv        : TEXCOORD0;
                 float4 pclip    : TEXCOORD1;
             };
     
             
             v2f vert (vin i)
             {
                 v2f o;
                 o.vertex     = mul(UNITY_MATRIX_MVP, i.pos);
                 o.uv         = mul(_Projector, i.pos);
                 o.pclip     = mul(_ProjectorClip, i.pos);
                 return o;
             }
     
             float4 frag(v2f i) : COLOR
             {
                 clip(1-i.pclip.z);
                 clip(i.pclip.z);
                 clip(i.uv.x);
                 clip(1-i.uv.x);
                 clip(i.uv.y);
                 clip(1-i.uv.y);
                 float4 col = tex2Dproj(_Decal, i.uv) * _Color;
                 return col;
             }
 
             ENDCG
         }
 
     }
 }