For my scratch built models I use brass tubing which if you size it right can get a very close fitting prop shaft to tube. I generally thread the prop rod end so the prop can be fitted with a lock nut, as well as solder a brass washer so the prop has some back support when tightened. I then add a nylon thrust washer to the shaft before inserting into the prop tube. For lubrication I use a product which can be purchased at a swimming pool supply store designed to be used on O-rings and gaskets. This is basically the same consistency as vasoline and is water resistant.

Before inserting the shaft into the prop tube, i lube it with liberally with the pool grease, and then sprinkle powered graphite on for additional lubrication.
Slowly insert the shaft into the prop tube and add more lube if needed working it in and out to spread the grease within the tube.
At the motor end, add an additional thrust washer, and attach the motor coupler.

I have not had any issues with water seepage up the shaft when sitting in the water for an extended period of time, or when running the model.

I pumped vasaline into the tube on my launch, using a large syringe, i also epoxied my propeller to the shaft

but i am a newbie to this side of RC stuff, as normally a flyer, so if ive done, or given bad advice, then i will take one on my knuckles

steve

You should have a thrust washer at either end of your prop shaft to take the horizontal thrust as the prop revolves fwd or rev. This should be a close fit to the bearing and the prop shaft. Use nylon /brass or stainless steel. At one time Rizla cigarette papers were used to set the gap. You should also fit a lock nut between the washer and prop to avoid the prop from spinning off. Same at the other end if the shaft is threaded. This with thin grease/oil will help keep the water from seeping into your model. There is a more efficient method using a stuffing box on the shaft at the motor end but this may not be needed for your occasional use model.
Please ask if you need further info
dave976

a good amount of grease up the tube and water won't come up even if the inner end is under the water level 👍👍