float Wave(x, y, t){
  float w = 2 / _Wavelength;
  float phase = _Speed * w;
  return float(_Amplitude * sin(dot(_Direction.xz, float2(x, y)) * w + t * phase);
}

...
o.vertex.y += Wave(v.vertex.x, v.vertex.z, _Time.y);

float3 normal(float x, float y, float t){
  float w = 2 / _Wavelength;
  float phase = _Speed * w;
  return float3(
    w * dot(_Direction.xz, float2(x, 0)) * _Amplitude * cos(dot(_Direction.xz, float2(x, y)) * w + t * phase),
    1,
    w * dot(_Direction.xz, float2(0, y)) * _Amplitude * cos(dot(_Direction.xz, float2(x, y)) * w + t * phase)
  )
}

...
o.color.xyz = normalize(normal(v.vertex.x, v.vertex.z, _Time.y));