There are of course two different devices that can look similar, sending units that often only have one wire, but sometimes two, they of course usually vary resistance with temp.
Then there are switches, which usually have two connectors but some only have one, they work
by supplying a ground through the switch body if they only have one wire.
Some are normally open and are often meant for a light or alarm
etc., when the set temp is reached, they close and complete the circuit.
However some are normally closed too, they are often used as “Murphy” switches, they allow power thru for example an electric fuel pump
, engine gets hot, switch opens pump
shuts off, engine stops, and some engines require power to run so when the switch opens, engine stops.
So in other words you could have several different things, but as nothing different happens when placed in boiling water
It may be that boiling water isn’t hot enough or you have a bad switch, or even a sending unit.
You can test for a sending unit by seeing if the resistance changes with temp.
I assume your engine normally runs about 165F? If so I think I’d pick a lower alarm
temp myself. 212 would be for an engine that normally runs close to 195 or so, of course I’m conservative too.
Try using oil
and a thermometer and take it to 220 or so, everything has tolerances and sometimes a switch made for 212 may operate plus or mins say 5 degrees or so.