I was in Fire Island last weekend, and let me tell you that finding pool parties to crash is a full-time job. I succeeded, hitting three pools and one hot tub, but it required all my acumen, flirtation skills and Jedi mind tricks to make it in the water.
Fortunately for you, NYC's outdoor public pools are totally open—no invite required—starting on Wednesday, June 27. Outdoor pools will be open daily from 11am to 7pm, with daily cleanings between 3pm and 4pm. Look out for free swim programs at your local pool, like early morning and evening lap swim hours (registration required) and swimming lessons.
Be sure to bring a lock for your valuables, towels, and the last six months' worth of New Yorkers that you've been meaning to read. The season ends September 9, so start making your Little Children plans with the Patrick Wilson in your life now.