Lock nut is good, there should be one anyway, or else there is a good chance of unwinding a prop when you go in reverse. The idea is that the lock nut is wound up against the threaded item (usually another nut) that is to be locked - there have been real world cases where this was not understood with tragic consequences. Use of thread locking compound helps as well.
If the coupler at the other end of the shaft is threaded on, a locxknut will help there, as well. I have seen a fast moving boat lose not only the prop, but the entire shaft like that. The shaft unwound from the coupler, at which point the boat was being driven by its other two shafts and the drag through the water over the dead prop was enough to pull the now loose shaft out. Since there was no rudder in the way, it came right out, and was never found again.