Back in the old days when the transmissions were always the manual type, then putting it in reverse did lock the prop. Now most all of the transmission are hydraulic, so that trick will not work, and should not be done. Install a proper shaft brake on the shaft, but make sure to release the brake before starting the engine and putting it in gear. Back in my old days when I was delivering boats, and we had a shaft brake, I always hung the ignition key on the brake control, so I couldn't start the engine without remembering where the hell I left the key.