I've used this firm,
http://www.southern-nikonos.com/ I probably need to send in my Nikonos for a checkup. Turn around was pretty quick, IIRC. Note that I haven't done any
real diving with my Nikonos.
As I understand it for very shallow depths - a foot or two - and shooting in rain, etc, pressure testing is probably not needed. Much deeper and best have it pressure tested. Southern Nikonos has a lot of useful info, and is a good thing to read before going underwater.