You do swim qual during zero week, starting at 4th class and moving down. Everyone else stops at 2, while the SNAs/SNFOs do 1. In 2, you have to swim around the pool in combat gear, pull a buddy across, etc. The pack you have floats, which makes it relatively easy. 1 is made up of three parts: 1. demonstrate the three survival strokes (breast, side, elem back) 2. rescues (approach your buddy who attacks you, and swim him across the pool) 3. 250 meter swim, any stroke. The rescues get some people, as does the fact that they're pretty anal about stroke technque. Overall, its not too bad. If you fail, they send you to remedial 3 times a week until you pass.
-again, this is just for the Marine side... no idea how it applies to the navy. Good luck