What's the fix for the fuel guage?
 

Before the fuel gauge worked fine, then it went to full all the time. Now it reads empty all the time.

It's my brother in law's 21 sailfish that he let me use.

So what do I look for to see if I can get it working?

with power on disconect the cables in the sender, and place together...it should read full, if not the gauge is bad or does not have juice. if the gauge works, the sender is the problem.

The two wires coming from the sender should read between 33 ohms full to 240 ohms empty when you check them with a meter unpluged. If it reads ol on the meter there could be a wire off or bad sender. If it reads 0 or close to that it is shorted out. If sender is variable between the 240-33 ohms then I would check wiring to the gauge for power and loose connections.