OK, this has really gone off the rails, let's be very clear, NO Inline Six released by Mercury for production use has EVER has a thermostat. To tear the motor apart looking for one would be counterproductive.
Best maint on an I-6 is to do routing decarbonization treatments, ensure the cooling system is working properly by servicing the water pump/impeller periodically, and making sure the timing/synchronization is correct.
Here's some info on decarbonizing and Link 'n Synch procedures that should help:
I have a 3 cylinder 60 hp merc and the thermostat is located under exhaust cover. i don't have an inline 6, i was suggesting he look there its not hard to do