Run a second nut onto the shaft so that the end of the shaft is flush with the face of the nut. Then use a good quality centre punch, put the tip on the "join" between the end of the steel prop shaft and the lock nut (looking at it from astern) and strike the punch with a hammer. Do this in 4 or 5 evenly spaced points round the junction of the nut and the shaft. It will stop the nut loosening. To get it off you will need a good socket and long bar for leverage.