10.5mm is an excellent lens to use underwater. However if you are taking pictures of manta, shark or other relatively large fish smaller than a whale shark, it may be a difficult lens to use unless you can get closer than an arm reach to it. 12-24mm or in cases of smaller shark ie 1-1.5m, a 17-55mm or 17-35mm would be a better choice.
I generally use either 10.5mm or 12-24mm depending on what I want to do for wide angle and either 60mm, 105mm or 70-180mm macro for the macro end of things, again, depending on what I want to do. I used to use 70-180mm a lot but it is huge and now the new 105mm AF-S VR is going to be my main macro lens.