New York City Waterfalls, Olafur Eliasson’s $15.5 million quartet of temporary cascades dotting the New York Harbor, formally opened on Thursday morning with a ceremony at South Street Seaport. Officials billed “Waterfalls” as the city’s grandest public art commission since Christo and Jeanne-Claude flooded Central Park with saffron-colored fabric panels for The Gates in 2005.