I totally agree with Harriette
I've been diving for about 15 years and got into underwater photography about 5 years ago. Uunderwater photography is great fun but can also be so frustrating when you start to add strobes until you really understand the lighting.
Start with something very simple, most of the P&S cameras have something like a "marine mode" but get a good housing. Shoot on this setting in the highest resolution and then have fun playing with the photo in Photoshop to bring out the colors(the basic photoshop elements does a great job with underwater photos as does most other photo manipulation software)
I had a basic Pentax Optio 5 for many years and was very happy with the results. I purchased a Sea and Sea DX 860G Kit about 2 years ago and are very happy with the results although getting used to positioning the strobe is still a work in progress!!
As with any underwater camera you have to take great care of the housing, soak it in fresh water after every dive and make sure the O rings are clean and kept lubricated.
The slighted little piece of sand or hair in the housing will cause flooding.
Camera do not like sea water!!!!
There is a great FREE on line underwater photography magazine available at www.uwpmag.com