Hi Bernie, welcome to Paddleducks

Some method of being able to dismantle the drive system is always a good idea. You'll appreciate the extra time taken if you ever have to take things apart!
A splitpin can be used, but I normally use either Meccano bits or aircraft wheel collets to attach my paddlewheels. Meccano fits onto 5/32" shaft, but aircraft wheel collets can be got in a wide range of sizes to fit different shaft diameters..... Just glue the wheel collet/Meccano wheel onto the inner face of your paddlewheel, and tighten onto the shaft using the grubscrew.
Wheel collets can be got from almost any model shop that stocks R/C model aircraft, Meccano can be easily obtained via Ebay.
I hope that helps a little?
Regards
Eddy