When you want to know a bit more about underwater photography and videography, check out WetPixel. They provide up-to-date news, reviews of specialistic equipment, competitions and so on. You can even join diving expeditions! To be short: this is the-place-to-be on the web for the real underwater photography addicts!
You'll find communities and competitions here. Both are motiviting and inspiring. In some way participants form communities too ;)