Had a similar problem w an older boat and when I opened the cover found zero blades on the core. I used bent wire w hooks on the end to dig and fish out the missing blades. Hopefully you got them all.
On our newer 'old' boat I replaced the impeller and then two years later was enroute to a weeklong sailing vacation.... and noticed no water coming out of the exhaust
. I shut down and checked the impeller. Half the blades were gone. It took about half an hour to replace and get under way but I will definetely replace it yearly now. And I always have a spare on board!