You can take the drive from the cassette of a bicycle and use a twisted chain to a prop. So if you had a 7 speed cassette you might use the highest 5 gears to turn the hub, spokes removed, then have a second chain from the biggest sprocket which twists the drive through 90 degrees as it descends to a sprocket to turn the prop. You could do the same without twisting the chain to drive a stern wheel. Mine is 80 cm in diameter and about 40 cm in width. I know from my speed that it slips very little in the water but have no way to calculate it's true efficiency. It's worth remembering that pedalling will easily give you more power than rowing, though sliding seats allow leg power to be added to arm power.
I'm hoping to build a catamaran which folds in half! It will be 2.4 meters in length and 1.6 wide with a simple paddle wheel mounted just in front of the mid point. The aim is to be able to use a small sail for sail assistance in a pedal powered boat. Weight will be around 50 kg so it could be carried on top of a car but I live near enough to the water to pull it from home on wheels as I do with my first boat. I'll follow your development with interest.
In the first incarnation of my drive system on the first boat I tried a vertically activated fishtail. It was OK on the down stroke but on the up stroke it just bobbed the hull into the water. If I built something like the Hobie Eclipse I'd try a linkage driven vertical fishtail working right under the pilot which should prevent the bobbing problem. A shaped sheet of PIR foam sandwiched between 2 sheets of plywood or aluminium for the paddleboard part and you're in business. Boat building is one of the most fun things I've done.
Here's a blog post about my boat
https://mountainbiker.online/2021/10/21/returning-after-a-cold/
