Hello
I have used the Nikonos II,III, and V, for years. About 6 years ago I broke down and got a housing for my SLR. Soon after I realized that the housing was great for standard and macro but the Nikonos/15 mm could not be matched for wide angle. I now use a Fuji S2 but due to the limitations (sun bursts/blue saturation) I will continue to use my outdated Nikonos and a 15 mm for wide angle until a better CCD or better firmware is available.
The Sea & Sea 15 mm may not be quite as nice as the Nikonos but it is quite close and you would be hard pressed to tell the difference in most images.
The 15 mm (Nikonos or Sea & Sea) is a wonderful tool and actually this is the only lens I have used on my Nikonos since I started using a housed SLR for non wide angle shots. If you must choose between the 15 and the 20, get the 15! The 15 will do just about anything the 20 mm will but you have to get "really close". With the 15 mm I can shoot a huge variety of subjects ranging from a Anemone fish 6 inches in front of the lens to a manta ray on a single dive. The depth of field can produce images with a almost 3D appearance.
As others have stated for someone new to UW photography the Nikonos may not be the best. If you are starting from scratch a housed SLR is a much better choice considering that a 15 mm lens alone will set you back anywhere from $500 to $1600.
My suggestion, if you have a nikonos set up and you are already familiar with it AND you plan to stick with film, get a second hand Sea & Sea 15 mm. Then buy Jim Church's "Essential guide to Nikonos systems". That book has some great tips on using the 15 mm not to mention tons of other useful info regardless of what type of camera you use.
Last note: To truly reap the benefits of this lens, two strobes and long connecting arms is a must.
Sorry if this reply is a bit jumbled, I am writing in a rush.
Kindest Regards
Chris