Hi Peter,
When you get close and shoot a model with a wideangle or fisheye lens, it can make them look very unattractive. Body parts at the edges of the frame can look VEEERRRRRYYYY strange indeed.
Consider that most topsides photographers prefer a 28-70 AFS or a 50mm 1.8 or an 85mm (on a full frame camera).
With that said, underwater photography is closeup photography. Rather than setting up for a studio shoot w/ the model 6-8 feet away, we usually try to keep subject distances down to less than 3 feet.
If you are JUST going to shoot a model as the main subject, I'd recommend the 12-24DX or the 17-55DX, both rectilinear lenses.
If you're going to shoot the reef w/ the model as secondary interest, I'd use the 10.5 or the 12-24DX.
Cheers
James