Definition
Raw water is untreated water drawn from outside the vessel — seawater, brackish or fresh — and circulated through the engine's cooling and exhaust systems to carry heat away before being discharged, usually through the exhaust.
Raw water is the opposite of coolant. Coolant is treated, contained and circulates for years; raw water is whatever is outside the hull, passes through once, and goes overboard.
The circuit, in order
Seacock, strainer, hose, pump and impeller, heat exchanger and any oil or charge-air coolers, then the mixing elbow where it joins the exhaust. Every component in that list is a wear or corrosion item, which is why the raw-water circuit accounts for such a large share of marine engine faults.
Why it is so destructive
It carries salt, oxygen, marine growth, silt and sand. It scales, it fouls, it corrodes, and in warm Australian water it does all of that faster than schedules written for cooler climates assume. Brackish river berths add their own problem, changing the anode alloy the installation needs.
Where you meet it
In every cooling diagnosis, every haul-out, and every lay-up procedure — see how to check raw-water flow.