Port and Lens is very important if you are doing underwater fashion.
the 10.5mm is a little too wide (too fisheyed) and the 20 is probably not wide enough. Are you looking for full body or partial body? If partial a 20mm would be close or get at 17-35mm or the new 10-24mm if you are trying to get a full body shot.
Old subal ports were the smaller 6 in and the Fisheye at 8in.
The 6 inch port required diopters on most lens while the 8 in did not. Some dome ports create a false image that is much closer than the lens can focus.
Make sure you can get a deal on the ports as the 8 inch port retailed at around $2000.
Subal ports are glass with great optics.
Refer to this links chart for lens to port choices
http://www.subal.com/en/portselection/lenses.php?markennummer=3