Portraits of scuba divers can take many forms.Whether you are shooting subjects on shipwrecks, on reefs or silouettes in kelp, the key is framing your subject and composing the shot to enhance the surroundings. Many photographers like to use wide angle lenses to reduce the water column between them and their subject. A 20mm wide angle lense will capture a average size diver from about 4 feet. A predive breifing is important when working with "models" . It should cover basic hand signals and how you would like to execute the dive.
Portraits are not neccessarily "static" or posed. An excellent technique is to stalk divers the same way you would approach animal life. Divers who are more intent on the surroundings, rather than the photographer will yeild natural shots of divers diving rather than "smiling" with regulators.
Underwater Photography Basics
Underwater Photography Terms
Adventurers Dive Club
Adventurers Dive Log