Guys only?

US Coast Guard approved Class 5 deck-suit.

Look professional and floating face-up is easier.

EVEN if you're proud of your body because you're in good shape, I'd still recommend a one piece bathing suit; you may suffer a wardrobe malfunction and expose parts of your privates, and will be embarrassed every time you see the guys that saw you. And, unless there are adults present, sometimes the guy can get a bit raucous and deliberately pull down your top or bottom... that won't happen if you wear a one piece suit. But, it's YOUR decision. Good luck and have safe sun.

It really depends on body shape and how the suit fits one.

Swimsuit. You do not want a wardrobe malfunction.

anything to show the nips & camel toe

If you will be doing any diving wear a swimsuit to avoid any embarrassing moments.

Honestly, as a 13-year-old guy, try to wear a swim team swimsuit, or an average bikini, don't try to go over the top or try to be sexy... (god I sound like a perv). Don't sweat about it, nobody really cares. Also, keep in mind that you are going to a waterpark, wear something that'll stay on on water slides and stuff.

Remember, my opinion, not yours, this is your body, be proud of it.

whatever you want it's your body