It's hot right now so swimwear on top because the life vest provides a good deal of protection from cold water. I'd wear neoprene short-pants because that's where you'll be most wet and need to stay warm. Because it's so hot in July you can get away with skipping neoprene booties but be sure to wear a sturdy water shoe with a closed heal like Chacos or Tevas. Don't forget your sunscreen!
We wore regular swimwear (shorts/swim shirts), and my kids rented the wet suits. I rented only the "splash shirt," or whatever it's called, and then the boots for your feet. For a daytime float, it's perfect. Kids seems to like the wetsuits though, so they usually get the full gear.
They give you wetsuits -- fleece is good to wear under because it stays warm when wet. No cotton. Evenings can be cool so some sweats or fleece.
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