As a memer of the URNU, I can safely say the warrenpenalver is pretty much right. The only EXCEPTION is that you will do the standard Navy Swim test. It can be done anywhere, but at the beginning of the academic year they'll take you down to Dartmouth for a weekend and they normally hold it there. - But don't worry, its not too strenous - 2 mins dreading water, swim 50m (2 pool lengths) and climb out of the pool unassisted, all whilst in a boiler-suit type overall that tends to hold lots of water (extra weight). To be perfectly honest, it sounds alot harder than it is, and so I really wouldn't worry about it, especially as I'm pretty if you fail that its down to the CO's discretion to not care about it anyway!
You'll be fine. Join in September if I were you, I really wouldn't join mid-way through, you'll miss out on a lot. There is no reason whatsoever that would bar you from joining in your second year of uni, I know loads of people in my unit who did so.