I'm betting you have one of three problems, now that I think about it.
1. There is something in the impeller, very likely. (unlikely if you were not in shallow water).
2. If this is a new boat to you and someone had the pump apart, they technically could have left the key out of the impeller and its slipping (also unlikely but plausible).
3. The crankshaft on the output broke, I would hope you would know if this were the case. The reason I bring that one up is if you tie the pump up hard and fast enough something has to break, could be the crank or the input shaft to the pump. very very unlikely.....
Hope this helps you out.
BTW haven't heard the story but there is a "loader" bearing joke floating around...