Just about every gauge I've ever seen has a green wire coming out of the back of the gauge with a little "G" beside it. Run a wire from that little green gauge wire directly to the negative terminal on your battery. If it works, that's the problem. I've got a couple of pieces of long wire with alligator clips attached at each end just for that very purpose. I just used it the other day to track down a shorted out wire I managed to create installing some light fixtures.
You're learning about fixing boats exactly the way most of us have done. Keep up the good work and look for the simplest solution first. You're well on your way to becoming an "expert"!!