Run a search. I know this has been done to death.
There's several quals you have to meet, but it's a pretty reasonable ramp-up in intensity. Initially they're just figuring out if you can actually swim well enough to keep going; if not, you'll get pulled out of class and they'll work on you until you are ready. If you get tagged for not swimming well later on they'll give you extra instruction after your last class, which can suck.
First day is just a length of each of the four strokes (breast, side, modified back, and crawl); second day is the same thing with flight suit, I think, then they just start working on distance so you get better at each of the strokes. You'll jump off a 12' or so tower in the abandon ship position wearing flight suit and boots and swim underwater for 20 yards (?) or so. You'll practice using the survival equipment, and finally do a mile swim in flight suit using any stroke you want. I haven't heard of anyone actually failing the damn thing, and we had some pretty weak swimmers; I would not worry about it.