Short answer
Watch exhaust-water volume, not just presence. A failing impeller shows as reduced flow, a rising coolant temperature under load, a squealing pump, or black rubber fragments in the strainer. Inspect the impeller directly; if blades are missing, find every one before running the engine again.
You will need: Screwdriver, torch, gasket or O-ring, replacement impeller
An impeller is a wear item with a limited life, and it fails gradually before it fails completely. Catching it in the gradual phase is the difference between a ten-minute job and a heat exchanger full of rubber.
The four signs, in the order they usually appear
Reduced exhaust-water volume. This is the most reliable early sign and the one most owners miss, because they check that water is coming out rather than how much. Learn what normal looks like on your boat at a known rpm, and treat any reduction as a finding.
Temperature that climbs only under load. Cooling that copes at low rpm and runs short at higher rpm is classic marginal raw-water flow. See why an engine overheats only at high rpm.
Noise from the pump. A dry or worn impeller can squeal or chirp, particularly on start-up.
Rubber in the strainer or at the exhaust. Black fragments mean blades have already gone.
Inspect it properly
Close the seacock, remove the pump cover and look at the impeller in place. Blades that are set in a curve, cracked at the root, flattened or hardened are finished even if none are missing. Count the blades against a new one. Any missing blade is downstream — in the heat exchanger inlet, the cooler tubes or the exhaust elbow — and must be found and removed, or it becomes a blockage later.
Full procedure: raw-water impeller replacement.
When to stop
If blades are missing and you cannot account for all of them, or the pump shaft has play or leaks from the seal, the job has moved beyond a straight impeller change. That is a technician's call.