The thermostat is supposed to MODULATE the engine coolant
temperature - i.e closed when cold, then opens to the heat exchanger
(or to sea water
if not fresh), and then closing when it drops again - repeat. If there is no thermostat installed, the engine will take much longer to heat up (not good in the long run) and then once at temp. it's actual operating temperature will be whatever the HX or sea water
can take away - again, not optimal.
As far as gauges, no they were not standard, they only had idiot lights. Speaking of gauges - if you gauge doesn't work, how do you know you are overheating
Seems to me the best way to start is to get the gauge working - pretty simple, a sender, one wire, the gauge itself.