i built a 1 to 200 scale plastic kit and modified it for RC, i used a standard servo ( acoms i think) for the propulsion, it worked fine, the output shaft from the servo motor was connected via a tiny UJ to the piano wire shaft.... the shaft, prop and coupling all came form SHG marine.... easy for us in the UK !!!1
chuck there must be a supplier in the US who retails something similar....
depending on the prop type you could maybe use fine bore silicon tubing as a UJ for your propshaft, i have seen this done before....
did not operate well in my boat as the tubing kept twisting- causing problems, i think it was just too soft to transfer the required 'drive' but maybe a stiffer tubing would do the trick, good luck anyway !