Considering all of the views and no responses yet, I may be the only lunatic on here who agrees with you.
The shaft driven propeller
may be the best thing that we have come up with so far, but it requires a lot of energy. Nothing in nature propels itself that way.
The idea of a fin like a fish
or whale uses would probably require much less energy, although it will never be able to push a boat at planing speeds. My vague, untested idea that I'd love to model someday is to use a ferrous arm connected to a large flexible fin. An electromagnet and a counter spring to move the short radius back and forth, giving the end of the fin a longer sweeping motion. It could be powered with short electrical
pulses to the magnet. The idea being that with short dc electrical
pulses it might be able to propel a long,narrow boat at a couple of knots using very little electricity.
Like I said, I'd love to build a small model and see if it works.