Finally remembered why I never use grease on impellers and it has nothing to do with deteriorating the rubber.
On many inboard engines, gassers and diesel the pump seal is a ceramic seal, like a PSS shaft log, not a lip seal. A little drop of oil or grease in the seal and it will leak like Niagara Falls.
The Mercruiser stuff is all lip seals I'm pretty sure. Watch out for Volvos, Cat and Cummins.
You know she is pumping water when all the soap bubbles come up behind the boat