Fish don't have propellers and get along very well without them.
Fish are highly efficient at propulsion.
Fish are highly maneuverable. Fish do not disturb their environment needlessly. Face it, fish are the best swimmers, everybody knows that. The tuna swims with high speed and high efficiency, the pike accelerates in a flash, and there is the moray that swims skillfully into narrow crevices. Someday a clever fish robot will swim skillfully and intelligently just like a fish.