Well the Gauge goes in the Dash
The location on the Water Jacket for the Tube, or in some cases, Transducer, vaires. On older Mercury Inlines, there was a threaded Hole for the Fitting on the Water Jacket Cover between the Spark Plugs, and plastic Tube was fed up to the Dash and fitted to the Gauge. I put one on my inline in 84, shorty after I bought it.
In Hindsight, I wish they had offered a Horn, that would sound if Pressure dropped too far, as you will pay attention to a loud Noise quicker than remembering to be always watching the Gauges. You do have see what you are running over.
The old Thermostatless Merc used a Gauge that read up to about 10psi, the motors that had Stats, used a Gauge that read higher