I worked hard last fall to splash the 12 m for first time after boat sat for a number of years (approx 7). Most if not all the mechanical gauges did not work. I blew all the hoses out and still not working. There livorsi.

Also if replaced are most going electronic or staying with mechanical.

they should be easy enough to test....if its a psi gauge then just hook up some compressed air....if its a temp gauge then dunk the sending unit into some hot/boiling water. Although i cant see a temp gauge having a long enough sending unit wire to get from the dash on a boat to the engine...unless you have it in the eng comp.
On my last boat i had all electric but i did have a small threaded gauge for oil psi right at the oil filter housing so i can verify psi when engine hatch is open and motor running