I've always wondered why nobody has created something that looks like a catamaran with two rowing shells and a road bike on top directly connected to a prop. That would have minimal drag, an optimal body positioning for using leg muscle strength, and would be fun and intuitive to pilot (facing forwards, feels like biking, etc). The obvious downside is that height above water may be an issue so the catamaran would have to be wide, but it seems solvable.