Splish Splash is Long Island's favorite waterpark, which has been featured several times on the Travel Channel's top 10 waterparks. It is far out on the island, but worth the trip. The park boasts many thrilling rides, as well as some relaxing ones (the lazy river is a blast). Admission is on the pricey side, as is the food, but you can find discounts at local chain restaurants.
Lockers are plentiful to lock up your stuff, and I'd recommend a plastic money holder to bring around the park. The park has shows everyday that are very entertaining. The family tube ride is the best, definitely a must-do!
Splish Splash is definitely a fun, all day activity. If there's time at the end of the day, check out across the street--the enormous Tanger Malls is just feet away!