There is no shortage of info out there, I suggest checking out the digitaldiver and wetpixel websites. Locally in S.D. we have the Divebums website which has a cool photo of the week section, it is great for actually seeing pictures produced by various cameras.
One thing to consider is a strobe, even if you’re not starting out with something fancy you’ll want to have some room to grow into, so make sure there is a decent compatible arm/strobe setup available.
There are plenty of older model cameras available second-hand that are of excellent quality. 5 megapixels should be enough for almost anyone. My first decent camera was an Olympus 5050c zoom and in fact I’d still recommend it. I bought the camera used and later picked up a used DS-50 Ikelite strobe for $200, vice over $700 I recently spent on a DS-160 strobe. Just make sure you get everything insured through DAN or DEPP before ever getting wet.