Well, I answered part of my own question...

Took the water pump apart. All six blades of the impeller had broken in
half. Luckily, I found them all. Two were down the bottom of the cavity,
between the inlet plates.

MAYBE that's why it wasn't drawing any water (duh).

Ah, well, I never said I was a marine mechanic, just a cheap boat owner. (A
boat owner who's cheap, as opposed to the owner of a cheap boat).