Well the Canon Digital Rebel series will take great photos of fish, depending on what kind of lens you get. But if you don't know much about cameras it's probably more than you need. Try Canons higher end point and shoots. Their G series and S_ IS series are really powerful, and are about as good as you're going to get from a point and shoot digital camera.
I have a Rebel XTI, with a 50mm lens and it takes amazing photos. I also have a Canon S3 IS (older model of the S_ IS series) which takes good photos, great videos, and has an AMAZING macro capability (you can focus on things that are almost touching the lens!).
If you're still in school/college see if you can take a photography course. It will make you a far better photographer. If you're not there are a lot of books on photography that can help you become a much better photographer.