It sounds like a current overload for the speed controller which may activate a cutout to prevent heating and fire. Look at the current ability of the controller (search it on online) then put an ammeter in series with the circuit to see what amps are being drawn by the motor. Obviously the current draw increases when the prop turns in water due to the water resistance on the prop.
がいいねしました AlessandroSPQR and Bob the boilerman