The split scene is from a tutorial by Anthony Worrall.  The link I had is broke - I sent him an e-mail to see if the tutorial is available somewhere.

Plants are from 3Dplants.com.  DAZ toon fish with Blue Boy texture by Ellemat (available at Renderosity's store). 

Hard to see, but the fish on the right is the pink texture by Pokydots.  Fish on the left is using the texture from DAZ. 

How to do the split scene

Create a terrain the will be the water (make sure it is not solid).  Flip it upside down.

Duplicate the terrain.  Make one terrain negative.

Make a 2D face.  Make it positive.  It should be slightly longer then the terrain and as high as needed to go a bit over the top of the terrains.

Group the negative terrain and positive 2D face.

Apply a water material to the group and the 2nd terrain.