Sounds to me like the L.U water pump. But is your Merc Fresh or Raw water cool

In article outboats.com,
says...

Hi all,

I just commisssioned my 19 ft Wellcraft for the summer season. After 6
months of storage in the mid-atlanta, the engine started after a few
moments and everything to seemed to be running fine. After about 10
minutes bringing the water temp to a stable 160 degress, I reved the RPM
to under 4000 and moments after that the water temp heated excessively and
all circulation stopped. There was no unusual noised from the water pump
prior to overheating. I have an Aplha-1 NG V-6 175 mercruiser. The water
pump was changed last season. Any thoughts or suggestions are
appreciated. Could it be the lower unit pump/impeller or thermostat?
Thanks