I started with a human-powered craft, in which I used myself as the engine. I sought to reduce the number of parts to a bare minimum. Even pedals, a drivetrain, chains, sprockets, etc., involve too many parts. Parts are bad! I went through a number of variations. Click here to learn about why and how I worked on this problem.
Eventually, I settled on the fishBOOT, so called, because you wear it, like a boot. It has only one moving part: the entire hull. You rock your weight, forward and back, which causes the hull to oscillate. It has a fluke, attached to the hull with a flexible tendon. Oscillation of the fluke generates thrust and the craft goes forward. The drivetrain basically consists of one moving part, plus the tendon.
It is easier to do than stand-up paddle-boarding. I am working on a new one-person version, which combines aspects of the one here, with the flexible hull of my two-person version, and a new steering system.