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.
And also CC Mr. Mercury (may be the most appropriate plugin in this situation).
To get general impression how to use this plugin watch Andrew Cramer’s tutorial Water Drops http://www.videocopilot.net/tutorials/water_drops/
Also you may want take a look at Harry J. Frank set of projects ”Best of Particular #1” on his Downloads page. There is a one among them called Fizzy. It does not look quite like fountain, more like boiling water but after some tweaking you may get something similar. Note how he used water drop created with Foam as custom particle.
And also may be Fractal Noise. And also if you have access to ParticleIllusion, check its emitters, I’m not sure but I think I saw something in there.