School of Fish is a tutorial that explains how to create a complex oceanscape, complete with a realistic water texture, beams of sunlight plunging through the water and bubbles floating to the surface. Plus, create a small school of fish that move individually and are different sizes.

This tutorial uses some fantastic plug-ins from Trapcode - Particular and Shine. If you don't own these plug-ins, there are demos available.

FYI, Toolfarm sells bundles of the awesome Trapcode plug-ins as well as individual licenses. Some even come with training.