I use marine suitable grease. In particular it is CRC Sta-Lube water resistant marine grease the type used to lube boat trailer bearings Len

The silicone will contain acetic acid (unless you use the very expensive stuff intended for microwave circuits) so i say keep it away from the bearings. As long as you grease the shaft...white lithoum grease...well. & the motor end is above the waterline, then you dont need to seal it as you will get very little water getting thru the shaft tube into the hull.

I have bedded a shaft tube in silicone where it comes up thru the keel into the hull, however i made sure there were no electronics in the hull until the RTV silicone had cured...no servo, no ESC, no lighting switch, NO RECEIVER.

Had a chance to add 2 photos.

I use the same approach as Lew. I have installed a pillow block between the motor and the propeller shaft and added thrust washers at each side of the block where the shafts enter and exit. The thrust washers take up the force from the propeller shaft in either direction of rotation and eliminates some of the strain on the end of the motor. I am in the process of putting together some build pictures but they will not be ready any time soon, Sorry too many projects, not enough time.

I just made up the prop shaft, and wanted to support the prop shaft inside the prop tube with bearings, just was checking water ingress thru the tube. Boat is going to be a1200mm Fairey Huntsman

Roller bearings? How big is this boat? Is it for high speed?

I've never used any bearings on any of my boats. All my shafts are scratch built using a brass tube with a smaller/short section of tubing inside each end to for bushings. Inside that is the prop shaft which is either solid stainless or brass rod. Near the motor end of the tube there is a "T" connection (short perpendicular tube) for filling the grease inside this assembly (called a "stuffing box"). Never had a problem and the shafts turn smoothly.

Here is someone's video that shows the process: Note that i don't use the grease fittings but do use a short section of brass tubing as explained above.

Lew