I would suggest that you consider the Tokina 10-17 fisheye like the above did. This is not a lens that one would really choose to get for above water shooting. But it has a number of very nice characteristics that make it excellent for underwater photography.
It has a very close focus distance, something like one inch. That means you do not have to have a diopter on it. It also means you can use a 4" dome with it which makes your set up fairly compact. If you want to do over/unders, you might want to go with a larger dome. The Tokina is a really nice lens. It is awfully wide with a 17 mm as your long part. It is great for landscapes, diver photos and even photos on the boat (surprisingly enough). On my last dive with it, I took a few boat photos with divers and was very surprised at how well it worked. On the down side, unless you are taking photos of very large fish that can be approached quite closely: nurse sharks, stingrays, turtles (yeah they are not fish but they are wild and underwater so they get included) and others, it is hard to get close enough to a fish to get a good photo of it.
A more versatile lens is the Sigma 17-70 macro. Again, like the Tokina, it has a mix of characteristics that lend it to underwater photography. It has to be used with at least a 6" dome or better an 8" dome. With the 17 end, it works as a landscape lens or for taking diver photos. Because of the pretty wide range of zoom, it works very well for fish like angel fish. It also has a close up ability but not true macro.
It really depends on your style of shooting and what you prefer to do. If you really like taking shots of fish with the ability to do landscape shots, you might prefer the Sigma. If you are into a dedicated wide angle lens for landscapes, diver photos, wreck photos, and on board boat photos, the Tokina is the way to go.
