Obviously, first and foremost, don't forget your swim attire!
Depending on where you're going, you can rent a device, but buying your own will save you money in the long run and also give you the chance to float more exclusive places without having to worry about paying for accomodations.
PACK. THAT. COOLER. My go-tos: Water, beer, fruit, protein.
For me personally, these are def MUST HAVES and I don't float without any of them.