Gallery: Wet: Macro
"Macro" shooting generally involves precision shooting very small subjects, or taking close-up shots detailing parts of a larger subject.
The challeges of shooting Macro underwater involve surge and current - when you're moving and your subject isn't (or vice versa) things get interesting.
My favorite Macro subjects are Nudibranchs - colorful and small (1/8" to 4") sea slugs that are found in most waters throughout the world.