There are all sorts of ways to ply the waters of New York Harbor. The latest? The increasingly popular Hawaiian-born love child of surfing and kayaking known as stand-up paddleboarding. Starting May 19, New York Kayak Company will offer open-paddling sessions on Wednesdays from 6 to 8 p.m. off Pier 40 ($25 for board-and-paddle rental; West Side Hwy. and Houston St.; 212-924-1327; bring a padlock). Two-hour classes will be held Tuesday evenings ($100; starts May 25). A few hours in the current- and wake-lashed Hudson should confirm that SUP, as the sport is known, is as exhilarating—and as tough of a workout—as it looks.