It usually starts small: a needle that won’t quite settle where it always has, a faint alarm you half-hope is a loose wire, a little less water out the exhaust than your eye expects. Out offshore, that’s enough to get your attention without making a fuss.
The trick is to buy yourself time and choices. Heat builds fast, and guesses get expensive. Before you start tearing into things, you want the boat under control, the load eased, and your own head clear enough to notice what’s changing and what isn’t.
Cooling systems fail in untidy ways. A bit of weed, a tired impeller, a sticky thermostat, a collapsing hose, a clogged elbow—any one of them can look like the others for a few minutes. The sea doesn’t care which it is, but your engine will.