Take a small boat, one or two people, WITH a paddle, and welcome to the Hudson River. Kayaking is a great, lazy out door sport. Unless you are racing, you won't be burning many calories.
Kayaking is cheap and remarkably easy. If you want to be more outdoorsy but don't want to overdo it, kayaking is for you. It's cooling, you wear a bathing suit, and it's different - so it makes a great summer date. It's the only 'water sport" I tried that doesn't require any muscle so I said it isn't exercise.
You need to be willing to get wet, you can do it even if your legs don't work, and in Manhattan at least, it's free, at Pier 26. They even give free lessons and a free locker (please bring a lock). It takes almost no effort, and requires minimal skills (besides swimming).
If you go to Pier 26, you will want a bathing suit and a T shirt. They provide a life jacket. You must know how to swim. Gender balance is fairly even and they take all ages. Younger children must share a boat with an adult. They have successfully accepted people weighing 400 lbs.
I would wear water shoes, but they are not necessary.
Each session lasts 20 minutes, but if it isn't busy you can do more. Weekend times are 9-5, and during the week they are open from 5 to 7:30. (last boat enters water at 7:10).
From May 21 to Oct 10, Kayaking is free from Pier 26 at the Downtown Boathouse. http://www.downtownboathouse.org/
If you like kayaking, you can buy a kayak and life jacket for relatively cheap, and go kayaking wherever and whenever you feel comfortable. I have taken trips from one lake to another (only problem is bringing the kayak back to the car....).