Searching this site and Googling is your friend. There is LOT'S of info on this subject and nothing has changed in decades of diagnosing a boat's fuel gauge.
Basically, you have the sender... the wiring to the gauge... and the actual gauge (and it's power/ground) to check out.
- 12V to the gauge and the ground is good?
- Remove sender wire at the sender and, with key ON, gauge should read empty. Touch the wire to the tank ground and gauge should read full.
- If still nothing, do the same test as above, but at the actual gauge.
- Ohm the sender... with your full tank, it should read approximately 30ohms.
Based on above results, you can narrow down to which of the three culprits is bad.