StiX floats are the easiest way to lighten up a heavy DSLR system underwater. Made by 4th Generation Systems, they are easy to put on and can be cut away as necessary.
Ultra-wide fisheye lenses are the preferred wide angle lens underwater due to their close focusing ability, sharp corners and wide field of view. Divers can usually get very close to their subjects with them, filling the frame, eliminating water which makes their photos lack sharpness, color and definition.
But there is a drawback to fisheye lenses; when shooting the straight lines, commonly of a pier or wreck, or models or other similar subjects, optical distortion is introduced.