Wait, so you can see the driveshaft turning and visually verify that the impeller is not or are you just assuming the impeller isn't spinning because the ski isn't moving?
If the impeller is actually not spinning but the driveshaft is, then there are really only two possible causes. Either the splines are damaged at the impeller or the driveshaft is broken. It's more likely that the splines are damaged or for some reason not engaged.
Thanks for your reply,
To reiterate, when the engine is turning the shaft from the back of the block right through the hull to the impeller is turning, what is not turning is the impeller. I assume from what you just wrote that the splines on the impeller have rounded or possibly the captive nut is loose. How do I get to the impeller to check? From the rear or from underneath?