Most photographers underwater use 2 types of lenses:  1) A wide angle lens. 2) A macro lens . As the water has a natural optical magnification , fisheye lenses are less circular underwater and allow taking wide angle shots from a close distance thus decreasing backscatter and water dirt. Therefore these lenses are very popular, the most common one is the Tokina 10-17mm.

For the same reason , the most popular Macro is 50mm which acts almost as a 85mm lens underwater.