Thank you all for your opinions about my raw water cooled Yanmar.

If I just let it do what it's been doing for thirty years, eg, running
below 120, Is it bad to let it run at idle for long periods? I would
like to use it as a generator sometimes while I'm using my 1500w
inverter or just to charge the batteries. I had previously been under
the impression that it was a bad idea to run it for very long without
engaging the prop because the motor runs so cool, so I tried not to do
that much.

After observing the temperature gauges (installed and manual) it seems
that running it engaged doesn't really warm it up much! Crazy.

So, bad to run it at idle much?

Stephen