Re: hot 7.5 Merc.
Does the engine show a good "pee" stream? If not you may need to replace the impeller. If it has a good pee stream, the other thing to check is the thermostat which may have stuck closed. If you do replace the impeller, while you have the lower unit off, clamp a garden hose onto the end of the water pickup tube while the thermostat is out and flush the engine.