It is quite simple. Do a web search for under water photo equipment for the latest gadgets as well as the time proven ones. Like the Nikon Nikonos.
The same technology that is used above water is used below water. The only new "stuff" would be water proofing means etc. Light is the same issue to deal with. Subject matter. Equipment operation.
I have used a $8.00 disposable "waterproof" camera with very impressive pics. But just like the above water equipment to advance to greater capabilities you need better equipment.
And of course you need to be a diver. With all that gadgetry.
My diving experiences and experiences in photography suggest to me the biggest issue in this equation is not the equipment. But getting a person who is a real photographer and a real diver. Over all probably a thrilling pain in the rear way to take pics for most people. But the subject matter underwater (salt water) can provide just awesome photo souvenirs to bring home. DO NOT DISCOUNT A WP DISPOSABLES ABILITY to bring home those snorkeling souvenirs! (I liked the Fuji)