Each engine has a raw water pump.
The pump is a plastic housing, and the impeller is made of rubber.
It your run the engine with no water flowing thru the pump it will melt the rubber impeller and the plastic housing.
In the water, the boat cubic weight displacement, will just about push water from the sea upto the raw water pump, making a worn out pump work.
If your not getting any hose water out your exhaust, then its shot... $80 for the kit for impeller and housing.
Not likley a clog or stuck thermostat.
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