I have a temp guage and I just hooked up a pressure guage since I have an
overheating problem that is pressure related (Mercury is working on it but
that's another L O N G $800 story)
In retrospect I think the pressure guage might be more important than the temp.

I only wish I had the pressure readings when it was working right.

There is usually a water jacket port where you can connect either water
pressure or temp. It should be up near the thermostat.

On my old 2 stroke 75 I had it simply touching the head, mounted on a bracket
on a threaded boss between #1 and #2. I had a blob of heat sink compound under
the sensor and it worked pretty well.