Camera, Housing, and Strobes
Unlike taking a photo on land, capturing an underwater photo requires a camera in an underwater housing and, usually, strobes to provide artificial light.
David Doubilet
Doubilet uses digital Nikon cameras (D3x among others) and lenses with Nexus and Seacam housings and Sea & Sea strobes.
Norbert Wu
Wu uses the digital Nikon D200 camera with an Ikelite D200 housing and Ikelite DS125 strobes.
Stephen Frink
Frink uses the digital Canon 1DsMKIII and Canon 5DMKII in Seacam housings with Seaflash 150 strobes.
Amos Nachoum
Nachoum still uses exclusively film cameras, primarily the Nikon RS, Nikonos III with the 15mm lens and Nikon F100 in a Sea & Sea housing.