How raw-water cooling actually works
A raw-water cooled engine uses the lake itself as coolant. A pump driven off the driveshaft pulls water in through the lower unit or the drive, pushes it up through the powerhead or block, around the exhaust, and then back out through the exhaust and the tell-tale. There is no reservoir, no radiator and no cap to check. The only visible evidence that any of it is working is water leaving the engine, which is why a weak tell-tale stream is treated as an emergency.
Because the coolant is the lake, whatever is in the lake goes through the engine. Sand and silt stirred up in the shallows wear the pump housing. Weed and debris block the intake screens. Lake Winnipesaukee is fresh water, so an engine here does not fight the salt scaling that a coastal boat does, but silt, sand and seasonal weed still find their way into the passages, and a boat that spends its time in shallow water near shore picks up more of it.
The impeller is cause number one
The water pump impeller is a rubber vane wheel spinning inside an offset housing. The vanes flex as they sweep past the narrow part of that housing, and the flexing is what moves the water. Rubber does not like heat, age, or being left bent in one position for months. A single run without water, a few seconds on a flush or a start on the trailer, is enough to glaze or shred the vanes. When a vane breaks off, the fragment travels upstream and blocks a passage.
A short season is what kills impellers here. The lake ices over, ice-out is usually declared somewhere around mid-April, and most boats then go back on the water for a handful of months. An impeller can be six or seven calendar years old with very few engine hours behind it and still be hard, taken a permanent set, and cracked at the roots of the vanes. Age matters more than hours. Follow the engine's own service schedule, and count years rather than hours.
The tell-tale, thermostats and blocked passages
The tell-tale is the small stream squirting out of the side of an outboard. It is a diagnostic rather than a cooling component: a bleed off the water jacket that shows water is circulating. A strong, steady stream is good. A weak stream, an intermittent stream, or a stream that is hot to the touch means flow has been reduced somewhere in the system. No stream at all means the engine should be shut down immediately, not run back to the ramp on the hope it holds.
A blocked tell-tale tube is a real and common cause of a missing stream on an otherwise healthy engine, which is why the check is temperature as well as flow. Thermostats fail too. They usually fail closed, which cooks the engine, or stuck open, which leaves it running cold and rich and carbonising the combustion chambers. Corroded thermostat housings, sticking poppet valves and silted passages in the block all cut flow without any single part looking obviously broken.
What an overheat actually destroys
Running an overheating engine destroys expensive parts quickly. Aluminium pistons expand faster than the cylinder walls around them, so the piston scuffs and then seizes, leaving vertical scoring down the bore. Head gaskets let go. Cylinder heads warp out of true. On a two-stroke, an overheated cylinder can hole a piston crown outright. None of that is repairable with a new impeller afterwards; it becomes a powerhead job. The temperature alarm is not a suggestion to be argued with.
Exhaust components take the damage nobody thinks about. Rubber exhaust hose, the exhaust tube and the water-jacketed housings all rely on cooling water flowing through them to survive exhaust temperature. Run dry, they scorch, harden and crack, and then exhaust gas or water ends up somewhere it was never meant to be. The sensible response to a temperature alarm is to shut down, check the intake for an obstruction, and get the boat towed rather than run the last mile.
Closed cooling on inboards and sterndrives
Inboards and many sterndrives use closed cooling, which is a two-circuit system. The engine runs on a sealed loop of coolant and antifreeze, exactly like a car, and that loop is cooled by lake water passing through a heat exchanger instead of air through a radiator. The raw-water side still has a pump, still has an impeller and still fouls with silt and weed; the difference is that the engine's internal passages stay clean. Half a system being closed does not make it maintenance free.
Closed systems need their coolant tested and replaced, because the corrosion inhibitors in it are consumed over time and the block depends on them. Heat exchangers silt up and need removing and cleaning out. The raw-water pump on these installations is usually belt driven and mounted on the engine, so its impeller is reachable without dropping a drive. Overheating on a closed-cooled engine can be either circuit, and the diagnosis starts by establishing which of the two has stopped working.