I wonder what the efficiency is compared to a screw propeller. I suspect less, but of course that's not the point. The use of stronger muscles in a more efficient way will help, like recumbent bicycles.
I have borrowed a friend's Hobie kayak a few times. It has a different pedal mechanism and when you push the pedals the blades move from side to side. It was much less tiring and leaves hands free to take photos, wave or eat sarnies. Also no drips running down the paddle onto your clothing.