My air conditioning has been frosting up and one of the kind forum members here mentioned that it was likely that the water wasn't circulating properly. This turned out to be bang on the money because over the next few days the pump started to struggle and not much was coming out of the through-hull. Time to investigate. I opened up the strainer to see if it needed cleaning but it seemed pretty good. Then I closed the strainer and turned the system back on - the pump sounded like sand was being poured down a pipe. This I recall having been told once is what cavitation sounds like and so I started looking for a way for air to get in. I looked suspiciously at the strainer lid.

These strainers are old cork gasket jobs and I'm sure they're in need of some new cork. Good job for the winter, but for now I gave a twist on the lid wingnut with some pliers and the cavitation sound improved some but not all. I cranked again and "snap" - the wingnut stripped the thread and I lost my ability to refrain from swearing.

Found a solution though: Then I noticed the float switch for the bilge was floating but no pumping was going on. Looked like some crap™ was stuck in the pump so cleaned that out. Looked at the wiring and the connections were all taped together - so took that apart and found another mess. But look at this wire! Pink and brittle. I had to cut a long way to get to good wire. This is factory wire and is not tinned. Bummer.