Too many possibilities for interpreting the term WATER FOUNTAIN.
A drinking fountain? Or something more, umm, architecutral?
http://www.fountainsinthecity.com/visit/famous_fountains.html
After Effects is not a very good fluid simulation package, even with a bunch of expensive plugins. But Foam, Particle World, Caustics, and WaveWorld, along with some fractal movies, can create excellent water simulations.
The Trapcode lobbyists will chime in soon, give them a few hours to organize some suggestions and links for you.
bogiesan