This is the way I do it: First, create a "Volumetric Slab" with a water texture and place this on top of your terrain so it touches the bottom plane. Next, create a cube, give it negative attributes, apply a thin glass texture to it and place it into the Volumetric Slab so it just sticks up above the surface. Group the cube and slab and place the camera inside the negative cube so you can see a cross section of water and sky. I find the volumetric slab works better than an infinite plane for this because it holds the texture all the way to the sea bottom. Use some terrains with marbled cloud textures applied to create splashes. Spheres with glass bubble textures add a nice effect too.