Generally, pegged to empty is an indication that it has lost it's ground, and or, a poor connection at the sending unit.
The first thing you need to do is look for a bad connection then trace the wire(s) to see if it has become open or frayed and shorted to a ground or the feed (hot) wire itself.
If I had to guess, the gauge is reading what it thinks is correct.
My guess is that you may have the polarity reversed on you wiring. You might want to change your positive and negative leads at the fuel tank sender or perhaps at your gauge.
Both suggestions sound reasonable, also there are two guages one uptop on the bridge and another below. There are a lot of crimp connectors in the wiring which is not great in marine applications.
One other possibility: The sender inside the tank may be fouled and stuck in the empty position.
The sender is a mechanical float that slides up and down on a plastic rod. Varnish from gas can build up over time and cause it to stick. If you can access it, unscrew it, pull it out of the tank and inspect it. Soaking it in kerosene or some type of carb/fuel system cleaner will disolve the varnish.
Be careful about flame sources (don't smoke!) while working on this problem.
I'm having the same problem on the Sea Ray... The gauge reads empty all the time. We tested it, and the gauge is working, so the problem is with the sending unit. (Don't ask how it was tested, as it was done by my neighbor, who's an engineer.) Now I need a new sending unit and the time to change 'em out!