[chumbucket]

First thing you need to determine is if you have voltage going to the sender. The problem is usually a bad ground or broken connection. Is there a fuse off the buss panel? Is there a switch on the helm? All these need to be checked and traced. Can be accomplished simply by either making a 12 Volt DC test light using a 12VDC bulb and soldering a couple wire leads to the base. Or purchase a cheap voltmeter or ready-made test light. Start from the buss panel and then work your way to the switch making sure that the switch makes good contact when thrown to the on position, then on down to the sender. One of the first things I do when trouble shooting electricals is to remove the wires and clean the terminal ends with a piece of emery cloth and re-install with a dab of vasaline on the terminal end to help prevent corrosion.
A good multimeter is handy to have when working on boats. You're going to run into all sorts of electrical glitches over time when exposed to salt air. This'll get you started, then I'm sure you may come up with a couple more questions. ;)