Pack Light — Leave valuables at home. If you're wearing your swimsuit, pack dry clothes for the ride home.
Parking Location — Write down where you parked your car and tuck the note inside your beach bag. With so much fun to think about, this would be an easy one to forget.
Sunscreen — When you're running around having a ball, it's easy to ignore the effects of the sun. Apply waterproof sunscreen generously and often. Remember, this applies to a lot of indoor waterparks, too!
Footwear — With literally acres of waterslides to experience, your feet will be on the move. Swim socks are a good investment, because you can keep them on during the rides. Or bring along flip flops that can be stowed in special shelving units set up at the base of most rides.
Sun Glasses with Strap — Give your eyes a break. Sun reflecting off water makes sun glasses more than merely a fashion statement. They're great for people watching too.
Swimsuits, Cover-ups, Towels — No directions required.
Quarters — Handy storage lockers for all your other essentials are plentiful. All they require are a few quarters. Nice value!
Extra Set of Car Keys — After a long day of fun, no one wants to find they've misplaced their car keys.