Prop spins when out of water but when i put it in the water it slows down to a slow creep

Yes very common, have you hit a rock as some stage or maybe it is wearing out but as soon as some pressure goes on the prop the shaft will spin normally but the prop is now no longer held to the shaft correctly hence your problem. Check it by marking the prop and shaft with a straight line then put it in the water, take out and check your mark they probably wont line up anymore. Solution...get a new prop. I have a different way to get these fixed but it is not recommended I drill a self tapping screw into my props just until it hits the drive shaft then stop just enough pressure to keep to stuck to the drive shaft