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Why not increase the size slightly and put a nema stepper motor inside the cone of the propeller? As long as you have the motor housed such that water will pass completely over the motor housing without getting in then further waterproofing shouldn't be necessary. Also, the bearings and shaft of the motor should be sufficient for this design so that you dont need to add external bearings (especially if the water is directed more/less evenly onto all the blades at the same time).