Physically, it's not the most challenging thing in the world. Remember it is a 'taste' of Naval Academy life, not the real deal. I expect it to be 100x difficult when I go for real, but NASS challenged not destroyed me physically.
The PT test is alot easier than the CFA, at least for me. Sea Trials is fun as anything! I loved it the most of any event we did. And I brought the list they told me to, and wore nothing.
I would just bring underwear, socks, shoes, and toiletries (your basics plus alot of sunscreen etc). They give you shorts, shirts, PT shirt, hat, water bottle, ID, paper, pen, folder, and.... anything else NASS veterns? I can't remember off the top of my head.