We put it in the locker unless someone was going to hang out at the chairs the entire time. There's also those waterproof bags like where you can put a cell phone and then wear it around your neck. He also has a Dexcom and it sends the numbers to his pump and phone. He usually wants to put the phone in a bag and take that instead of the pump.
We also find that he tends to have more trouble going low at a water park. We grab the pump when we eat and he gets it sometimes to do corrections if needed. He's managed to go without basal all day by giving himself insulin every few hours. YMMW it really depends on the person and what they're eating and activity and so on. If you don't need to carry a cell phone I'd put the pump in one of those phone bags and hope it works. We bought ours last time at Typhoon Lagoon and still use them!
You could also get a basal insulin from your doctor if you think lack of basal is going to be a big problem...and use it for the day.