Gotta-Have Gear: To be an underwater photographer, you obviously have to have an underwater camera system. You also need to be properly equipped and trained for the type of dive (tropical, temperate, ice, tec, etc.). And you need to have the basic skills under your belt for that type of diving before you take a serious camera system along.
Plus Gear: Laptop computer with portable, photo-quality printer. With almost all serious underwater photography digital, it's a real plus to be able to download and check what you shot right on the boat or back at the hotel. Don't forget to Photoshop.
Gotta-Have Non-Diving Accessory: Hat or ball cap. I don't know how many times I've seen people lean over their open camera housing between dives, only to drip seawater from their hair onto something delicate or expensive. Usually they luck out but a better option's to dry your hair and then put on a cap so it doesn't happen.
Gotta-Have Movie: Blue Water, White Death, the cinema documentary about Peter Gimbel's search for the great white shark. Released in 1970, about five years before Jaws , Blue Water, White Death follows four of the giants of underwater imaging (Peter Gimbel, Stan Waterman, Ron Taylor and Valorie Taylor) as they travel around the world filming sharks. Good luck finding it, though watch for it to air on late-night television.
Gotta-Have Book: Manfish with a camera by Jerry Greenberg. This short work details the early 1960s career of Jerry Greenberg, one of the earliest divers to successfully make underwater photography a full-time profession.