Not disagreeing, but just wanted to point out that if the goal was to generate heat then humans could potentially be more efficient because of internal efficiency. The ballpark number I often see is 25% efficiency, so if you were able to somehow capture the waste heat they'd be able to heat up the water 3x as fast.
This would probably work better when heating up rooms rather than water. The benefit of using electricity as an intermediate form of power is that it becomes possible to heat water up to boiling temperature. Exercising humans never reach boiling temperature no matter how hard they try.
Fair point, though that is arguably the only situation involving electricity (besides heating the room itself which doesn't involve heat capture) in which you're actually trying to generate heat.
