Unity surface shader not casting shadows on itself

I'm new to shaders and I'm trying to make a shader for some very simple low poly water. I have created this simple surface shader that changes the vertex heights.

The problem is that it doesn't receive shadows as you can see in the gif and there is a weird behavior on the edges.

What am I doing wrong?

Shader "Ramble/LowPolyWater" {
    Properties {
        _MainTex ("Texture", 2D) = "white" {}
        _Color ("Main Color", Color) = (1,1,1,1)
        _Emission ("Emission", Color) = (1,1,1,1)
        _ScrollYSpeed("Scroll y speed", Float) = 0.1
    }
    SubShader {
        Tags { "RenderType"="Opaque" }
        CGPROGRAM

        #pragma surface surf Lambert vertex:vert

        struct Input {
            float2 uv_MainTex;
        };

        sampler2D _MainTex;
        fixed4 _Color;
        fixed4 _Emission;
        float _ScrollYSpeed;

        void vert (inout appdata_full v)
        {
            v.vertex.z += frac(sin( dot(v.vertex.xyz ,float3(12.9898,78.233,45.5432) )) * 43758.5453) * sin(_Time.y) * _ScrollYSpeed;
        }

        void surf (Input IN, inout SurfaceOutput o) {
            o.Albedo = (tex2D (_MainTex, IN.uv_MainTex) * _Color).rgb;
            o.Emission = _Emission.rgb;
        }
        ENDCG
    }
    Fallback "VertexLit"
}

Figured it out!

I just need to add addshadow to this line #pragma surface surf Lambert vertex:vert addshadow