Some of the most interesting shots taken underwater are of small stuff. Macro photography is certainly a lot of fun underwater as small subjects are often very colorful and have interesting shapes/patterns. Here a few examples of macros (taken with the Nikon D2x and a 105mm Micro lens in a Subal housing) from my portfolio...
Harlequin shrimp, this guy is about 1 inch long and is feed on the sea start beneath it.
A pigmy seahorse, this is the smallest subject I ever photographed, less than 1/4 inch long.
Close-up of a blenny living inside a coral in Venezuela.
A maroon clownfish, these guys get big but you can get very close to them, they are fearless!
And my favorite macro shot, a blenny in Pacific Panama...
Photo credits: Luizrocha.