I was down on Saturday and passed along everyone's suggestions. FYI, I did ask the oil pressure vs. hi temp alarm question last weekend and this particular Yanmar control panel has an idiot light for oil and another that says "water". The water light was on when the horn was sounding. I don't think this is sophisticated enough to tell that there is a low flow issue, just over temp??

In any event, it was a non-issue. The owner had talked to one of the old timers at the club who told him it was probably and air bubble issue(??) I have never heard of an air bubble issue. The old timer said that sometimes when a boat is dropped in the water after winter storage an air bubble forms within the cooling system that a small diesel at idle can't overcome. There is not enough vacuum generated by the impeller at idle to suck up water and force the bubble through the cooling system. His remedy was to run up the revs to get things pumping through. The owner also did his due diligence to make sure the seacock was functioning properly, the cooling hoses were free of clogs and water was coming to the inlet side of the pump. All good. He started it, ran up the revs and it never alarmed. He ran it over to the mast stepper and back, all good. Let it sit at a fast idle for 15 minutes, still no alarm.

So, as strange as this sounds to me, I guess it was an air bubble issue?

Thanks everyone. I now have some useful troubleshooting information for the future.