Hopefully not to late, but like most answered wear something comfortable, pool party attire.
Last time I went to Nightswim, I was in a suit, . Granted, we had tickets to Light and plans changed at the last minute cause we found some friends with access and bottles. Let's just say that suit survived but had to be taken to the cleaners asap on Monday. Went in the pool in my underwear, smashed while David Guetta was spinning,