I personally don't use any bc his doctor told us not to until 6 months. So long sleeve swim suits and a hat or I just keep my baby out of the sun completely!
Take warm clothes. Even when you think it's warm it can be cold for the baby. But you should be fine. Just make sure if bby goes in the water wet there hair asap
I took my daughter to the pool for the first time around that age and she was perfectly fine. Obviously don't let baby drink the water and he'll be okay. I also didn't have her in too long. It was just really a quick dip a couple times and to get her feet wet.