Canon users have long loved the Canon EF 8-15mm Fisheye. Now finally Nikon shooters can get in on the action. The Nikon AF-S 8-15mm f/3.5-4.5E ED has a fantastic zoom range for underwater photography, with a circular 180 degree angle of view on full frame cameras on one end and a non-circular fisheye view on the other. This lens can be used on crop sensor cameras like the D7200, but won't provide the full circular 180º field of view.
The lens is compact and lightweight which is great for underwater travel. Several proprietary coatings help to resist fingerprints and smudges and reduce ghost and flare.
Fisheye lenses provide the sharpest wide angle images achievable behind a dome underwater, and the Nikon 8-15mm is no exception. This is a fantastic lens for shooting wrecks, reef scenes, and large animals. The 15mm end is probably the one you'll be using most, but the circular end can really capture that WOW factor with the right framing. If you're shooting models in a pool you may want to consider a rectilinear lens like the Nikon AF-S 14-24mm f/2.8G ED instead.