It's not so much the pump housing inside that wears (which I understood earlier) as it is the cam. Evidently, being metal and in contact with other metal, they can wear out, reducing pump flow (if that's my problem)...and I think the pump may be over 20 years old.

But, again, after I go through the entire system from seacock to elbow --once more!-- leaving no stone unturned, no corner undusted, no hose unblown-through...and, yes, a coolant flush...then I'll re-measure raw water output at exhaust and at pump to see if there is any change, and take boat out under load to check at the RPMs we usually overheat at. (I may just keep going, toward the horizon...). If the same, then the pump comes off and goes to a pump shop!

(I don't get why Efraim would contend that there's no reason to measure raw water output in two places --it seems obvious to me that comparing output per minute from the exhaust to output per minute from the pump would give a clear indication of any blockages downstream of the pump.)

Anyway, item #3 in the drawing below (I found my parts catalog with the nice exploded diagrams!) is the raw water pump cam. I gotta hold my peace on this whole issue until I actually get down to the boat and re-check everything again! Too easy for me to sit here and hypothesize...