The Hudson River meets the Atlantic Ocean in the vast, brackish New York Harbor, wherein the powerful southbound river current collides with even greater Atlantic Ocean tidal forces, sometimes pressing against and sometimes ebbing with the river current.
On the east bank, near West 58th Street, lies Pier 96, and on that are the new digs of the Manhattan Community Boathouse where volunteers allow you to borrow their sit-on shell boats, which they call kayaks. There you can experience some effects of those conflicting currents (and winds too) in a huge sheltered area situated between old, still existing piers.
It’s all free, on a first-come, first-served basis. If you declare that you can swim, you will be given a life jacket, a paddle, a ‘kayak’, and a locker for a session of approximately 20 minutes. Under 18s must be accompanied by a parent or guardian. An adult must be present with minors at all times, and children may not be left unsupervised. Under 16s must share a two-person kayak with an adult. You might get wet—dress accordingly. Changing rooms are available.
The closest public restroom is at DeWitt Clinton Park (West 55th Street and 12th Avenue). Pier 96 has no public restrooms. If you are arriving by bicycle, have a lock.
Keep your eyes open for wildlife—the return to the Hudson River of the Atlantic menhaden (a fish) has precipitated the return of their predators: humpback whales, ospreys, eagles, and others along with seahorses—good luck spotting one of those!