Find yourself a 12 point 3/8" socket and remove the top of the drive. In there you'll see a set of gears. If they look ok, then put the lid back on (screws torque to 20ft-lb)...
With the key out of the boat and in your pocket (and preferably, the battery neg disconnected too), put the control in forward gear. Now try to turn the prop CCW (yes, that's in the direction of reverse, but if you understand a racketing dog clutch, you know why. If you don't understand a racketing dog clutch, do it anyway.
) See if you can turn the prop. If the prop turns quite easily, then you have a sheared drive shaft or a spun prop hub. Mark the prop and the prop shaft. Turn the prop half a turn. Marks remain lined up, it's the drive shaft, marks move, it's the hub (buy a new prop)... If the prop turns, but is very stiff, then do the hub check, and if that's ok, then get a helper to watch the yoke (at the back of the engine. Access might be difficult) and see if that turns too. If it does, and the engine isn't being turned, spun coupler, engine out.
Chris......
