Did you have a thermometer in the pot of boiling water, or did you just boil water?
I ask as a thermostat in a boat
should open about 165 degrees or so, and can be bad, but still operate at 212f.
Either way, get a new one and while your at it a new gasket
Checking / cleaning
the heat exchanger can never hurt either, and flush and change the coolant
. Since you have to drain it and could well get salt water
in the fresh water side when you pull the heat ex tubes out, I flush and change the coolant
any time the heat exchanger is opened.
It only takes a tiny bit of salt water
to contaminate coolant, or so I have been told.