What engine?
If a volvo penta the raw water pump/impeller is mounted to the engine on the bottom starboard side of the boat. Remove the two hoses attached to the back, then loosen the 2 bolts that hold the pump on, remove the belt from the pulley, and remove the two bolts allowing the pump to come out. REALLY simple.
While you have it out replace the impeller as it is fairly inexpensive and incredibly easy to do. If your old impeller looks like it is still in good shape I would move on to the thermostat housing as the next possibility.
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