I think the 1st thing to do is replace the thermostats, they do have a tendency to have salt build up.
The poppet valve is located on the starboard side at the bottom of the block. The lower cowling will come off and make it easier to get to the 3 bolts that holds the cover on. Maybe go to Merc Express parts page and take a look.
ttp://www.mercurypartsexpress.com/Pa...00060224064276
The overheat is a constant BEEEEEEEEEEEEEEEP. Over heating in that range is common poppet sympton. About 3000 rpm, (as pressure builds) the poppet opens and dumps excess water thru the exhaust. Both fixes are fairly cheap for parts, although the EFI parts tend to be about twice the price of carbed models.