Reduce load and decide whether immediate shutdown is safe. Steam, burning smell, loss of exhaust water or rapidly rising temperature calls for prompt action. Never remove a hot pressure cap.
Raw-water branch
Check whether the seacock is open, intake is blocked, strainer is full of debris or air, pump belt is intact where applicable, impeller is damaged, hose is collapsed, cooler tubes are blocked, exhaust elbow is restricted or exhaust hose has failed internally.
Closed-coolant branch
Check coolant level only after cooling, external leaks, expansion-bottle behaviour, trapped air, thermostat, circulating pump, heat-exchanger condition and pressure cap. Persistent bubbles, coolant ejection, hard hoses from cold or coolant loss can require combustion-gas and pressure testing.
Load and measurement branch
An over-propped or fouled boat may overheat only near full load. A faulty sender, corroded connector or inaccurate gauge can create an apparent overheat, but verify it independently before continuing.
If an impeller lost blades, find them. If the alarm followed recent cooling work, check valve position, strainer seal, pump prime, coolant bleeding and hose routing first. After repair, test progressively from idle to operating load while monitoring temperature and leaks.