I hate having something on the boat that does not work. The fresh water gauge never worked so I worked on it and found the issue.
First I pulled gauge out to determine how it was wired and what model it was. It is a VDO SW 40/102 12V. Of course, nothing came up on google so I made a drawing.
Next I checked for 12V at gauge. Nothing. Well, that is an issue even if the sender is bad. So, voltmeter at hand and using my eyes for tracing wires I found the problem.
1. Gauge tied to a breaker with tape over it. Looks like it used to be for an anchor light?!?! Who knows.
2. Found corroded broken wire at the breaker.
Required everything and now she works just fine.
Pics and crude drawing attached for your amusement.