Fred-
I've sometimes had trouble with the fuel gagues on my older boat.
It's always been due to a weak connection at the top of the sender unit(s) on the top of the tank. The wiring up at the gagues in the cabin is much more protected and less prone to open circuits.
Get a VOM meter (volt-ohms-amps), and go back and see that you have power going into the unit by checking the incoming voltage against a ground, and then checking that there is voltage the return wire by checking it's voltge against a ground.
The most common problem here is simply corrosion on the push-on solderless connectors on the top of the sending unit. Water (particularly salt water) splashes up and is sprayed on the terminals of the sender unit that sticks out of the tank, corroding the connections.
Work gently on it to clean the terminals, as the corrosion may be so bad that they break off! In that case, you'll need a new sending unit.
You could also have a problem if the ground wire to the gauges is broken or off. I only have one gauge with a Port/off/Stbd switch and a single ground. If you haqve two gauges, they may still share a common ground. Check with the meter to see that the ground wire is connected by checking the resistance between it and another known good ground wire. There should be nearly 0 ohms. If it's higher, ground a wire between the known good ground and the gauge ground, and the connection should allow the gauges to work. Then fix or replace the broken/faulty ground wire.
As Tyboo says, "Ain't boats fun?"
Good luck and keep us posted!
Joe.