Unless NASS has changed from when my Mid was there, you should bring a swimsuit, but don't worry too much if you don't know how to really swim. They realize that lots of people don't know how to swim. They did NOT do any sort of swim test. They did do PT in and around the pool.
Even during Plebe summer, they did more water safety drills (drown-proofing) than anything. At the end of Plebe summer, my Mid had the chance to validate out of Plebe swimming, but wasn't quite fast enough. She did validate Youngster swimming, since that's a longer distance with a bit slower pace.