A spa party consists of more than stripping down and jumping in.
Please, wear board shorts.
A pair of speedos, or worse, your underwear, are not sufficient to swim in. A pair of board shorts ensures nobody sees more of you than they want to. It’s inevitable that everyone will be able to picture you naked, you don’t want to help them when it comes to the details by wearing a pair of underwear. Swimming in your Y-fronts is uncomfortable for everyone, except you.
Your board shorts should not be a relic of your childhood. Choose simple geometric designs or solid colours, rather than childish prints. I highly recommend purchasing your board shorts from a specific men’s retailer rather than a discount department store, as they tend to make their adult board shorts as giant versions of children’s ones.
It is essential that your board shorts fit well so you don’t have flesh spilling over the sides. To this end, ones that are secured with a drawstring are better than a zip or velcro.
Plan your outfit so you don’t have to wear socks, in case you forget to take them off before stepping into the water. You also need to wait until you’re properly dry before putting socks back on at the risk of suffocating everyone with your wet sock smell, so it’s best if you can just avoid socks. A pair of flip flops or sandals is invaluable at a pool party, giving you the mobility to step in and out of the pool as much as you like.
Wear a T-shirt or a short sleeved shirt
A pool party is as informal as it gets, so a tee will suffice. If you want to look slightly smarter, wear a short sleeved, collared shirt in a trendy print. Keep in mind that you will be taking your shirt off anyway, so it doesn’t really matter what you’re wearing top half.