It’s a paradox in our mostly sedentary lifestyles that when we actually exert physical energy, it’s wasted. What’s the use of visualizing your spinning class on the Tour de France if your bicycle doesn’t have wheels? I say, why don’t we hook up that iPod-wearing sweat machine to a plow, and get some work done? Unfortunately, many gym-going urban dwellers don’t own plows.

So here’s what happens when Mitchell Joachim, an innovative urban architect (featured recently on Earth & Sky) is related to a personal trainer: They create a gym that provides workout and eco-friendly transportation. It’s a gym that uses the power of treadmill-pounding, stationary bike-riding, elliptical-pedaling workout fiends to move the less-motivated around the city. And did I mention that it floats?

The “River Gym,” as it’s called, is a series of pod-like fitness islands that paddle around on 15 minute circuits around Manhattan. The idea is that the gyms would ease the load on ferries, and they’d also be outfitted with water purification devices to mitigate water pollution. Docking facilities would provide lockers, showers, healthy snacks, and the like.

So why just exercise when you can get an eco-workout? And why just take public transportation when you can ogle a bunch of sweaty people?

Sounds fantastic. But: Does this gym ship need a captain? I’m not sure what kind of traffic is on the Hudson and East Rivers, but I can imagine the tragedy when the gym hits a barge and everyone’s too tuned out on endorphins to care. And despite the high demand for exercise facilities before and after work hours, how will the gym be powered during off-times? Will everyone have to wait on the dock for someone to sign up for the elliptical in order to get across the river?

So the River Gym is good idea, that like so many innovative eco-designs, will never leave the port of the imagination. I’ll just stick to rowing a real boat for exercise, and keep developing my cutting-edge workout idea. It’s called “Lease Yourself to a Farmer.”