Most of the time a fuel gauge is an ohm reading device. The float in the tank moves up and down, the movement up and down creates more or makes less resistance(rheostat), thus moving the needle on the dash. Touching both wire together would be a direct short, 0 ohms and depending on the gauge would cause it to be empty or full. Is it a two wire or 3 wire tank sender

There have been issues with the rheostats on the senders getting corroded from ethanol based fuels. This is common in Audis and Vettes. I've had a buddy run Sea Foam through his vent and it cleared up the problem.

I think with a boat you can pull the sender and clean it with contact cleaner.

It's a two wire sending unit