As if there weren’t enough knockout parks in this city, the Babylon of New York playgrounds is set to open this Saturday, at Brooklyn Bridge Park’s Pier 6 (Atlantic Avenue and Columbia Street, Cobble Hill). The harborside green is sectioned into four equally stupendous play areas: There’s Swing Valley, which includes Tarzan-style ropes and double-seaters; Slide Mountain and its thirteen-foot winding tube slide; a 6,000-square-foot Sandbox Village; and the Water Lab, with a moat, a domelike fountain, and a wading pool. Later this summer, a free water taxi will make the five-minute trip back and forth to Governors Island.