I use an old Olympus C4040 with an underwater housing when I go snorkeling. I've used in several times in the keys, and even took it to Hawaii on vacation last year. Great setup, neutrally bouyant in the water, and easy to use. The camera has a fast f1.8 lens, so it's useful for ambient light shots in shallow water. Only problem is, I seriously doubt the camera is available anymore (it's my old P&S), and the matching Olympus housing was never sold here, mine is a grey-market japanese import that I bought on Ebay.
I would suggest one of two methods for assembling an underwater kit. First off, don't risk a brand new camera. Use something older, so you won't be too upset if/when you accidentally flood the housing. IMO, your best bet to put together an underwater setup is to purchase an old setup that someone is selling on Ebay. When I bought my Olympus housing, it was sold with an even older C3020 model camera for $300. I kept the housing, and resold the camera & accessories for $100.
The other option is to look for an older camera that has cheap underwater housings. Look for a decent hard cases with O-ring seals, not the cheapo ziploc bags that are so common these days. Be flexible with the camera choice and you should be able to put something together without spending too much.