The subs have alternating crews and the men serve each alternative time period subject to being at sea. My son will be joining the USS Ohio next month for his turn.
That's only true for the 14 Trident SSBN's.
Fast Attacks (SSN) only have a single crew.
I came across this on the Navy news web site and thought you'd be interested.
http://www.news.navy.mil/search/display.asp?story_id=27392