I break scuba underwater housings into two categories: those for use closer to the surface and those for use at deep depths. If you're a scuba diver and plan on diving a WWII wreck at 60 feet in the Philippines, you need a dive housing. Companies such as Aquatica and Ikelite make custom DSLR scuba housings rated for very deep diving.
Most photographers need a housing that works closer to the surface or snorkel depths around 20 feet. For this task, I use a sport housing designed for shooting on the surface and at shallow depths.
I shoot my Nikon D300S in an underwater housing by AquaTech. This housing is rated to a depth of 30 feet, but offers some notable advantages over other dive housings. First is its size. The AquaTech housing is very compact and easy to manage, much smaller than many dive housings on the market. Another huge advantage is the tight seal where the lens port attaches. Many dive housings aren't designed for getting pounded by waves as you photograph whitewater kayakers (or maybe your kids playing in the pool). Dive housings can leak water in rough surface conditions until you dive deeper and the pressure seals them tight. The AquaTech lens port screws into the housing and can take a beating in waves on the surface without leaking. Another big advantage of this housing is that AquaTech manufactures lens ports to accommodate telephoto lenses for shooting at the surface. Standard dive housings generally have ports for macro and wide-angle lenses.
Underwater FlashUsing flash underwater can dramatically improve color quality in your images. As you go deeper, the red spectrum of light is filtered out, resulting in blue/green images. Flash can restore the warm color balance to an underwater image. You have three choices for using flash underwater.
First, PVC housings from ewa-marine have models with vertical pouches to accommodate an on-camera flash. You attach your speedlight to your camera, put it in the PVC housing, and you're ready to go. You can access controls by pushing buttons through the PVC material. This is the simplest option.
Another option is buying a dedicated underwater strobe to attach to your dive housing. These flashes are attached via a waterproof cable to a sync terminal on the housing, and the cables are long enough to allow off-camera flash use. If you've tried underwater flash photography, you know it's critical to get the flash off-camera to avoid backscatter, which occurs when particles in the water reflect light from the flash, especially if the flash is on-camera. Imagine a snowstorm in your underwater image and you get the idea. By moving the flash off to one side, a lot of backscatter can be eliminated. Underwater flashes work in a variety of modes, including manual and TTL mode, but you won't have all the functionality of a dedicated speedlight.
There's another flash option, however. AquaTech manufactures a speedlight housing for Nikon and Canon flashes, allowing you to use your speedlight underwater with full control of all its features. This housing uses a waterproof cable to connect the flash to the housing, with enough extension to use the speedlight off-camera. Better yet, you can use a speedlight in high-speed-sync mode in the housing or attach a PocketWizard transmitter to trigger larger studio packs on shore. I frequently use this setup to photograph fast-moving whitewater kayaking.