They give you a basic swim test, administered by Navy SEALs. It involved (at the time, mid-90s) jumping from a 25ft high diving board into a pool, and treading water for a bit. This simulated having to dive over the edge of a ship in case you had to evacuate.
Anyway, I climb up, get to the edge of the diving board and just freeze. A very unhappy SEAL actually had to climb up and push me off the board. I must have been a really sad, pathetic case because one of the other SEALs doing the training pulled me aside and worked with me for the remainder of the day so that I could pass the swim test later that afternoon.
Kinda makes me wish I had learned when I was a kid, or had the time to learn now. At least I can still tread water.