I am no expert, no boat mechanic, but a pretty handy guy. Replacing the gauge and the sending unit are two easy things to do.
Easy to access them and change them out. My gauge was a Sierra, and was about $35. The sending unit I bought was a Moeller, and about $40.
There is a way to test which is failing (gauge/sender) , and I thought I did it correctly and it indicated the gauge, but it was the sending unit.
The gauge sits in gas, so yours has been in there for a while and I'd guess it's corroded, and I'd start there. I bought off Amazon, and made sure they were returnable.
When working with the sending unit, I connected the new one first outside of the gas tank. moved the float, and saw the gauge adjust.
I did this in case it wasn't the sending unit, and I could still return it. If I put it in gas, don't think I could return it. Now my boat is 8 hrs away in a vacation home, so I don't have handy access to it. To best order your sending unit, you need to know the size of your tank depth. Take the old sending unit off, and put a ruler in . Measure from bottom of tank to top exterior. Use this measurement to order your sending unit. I didn't know the size of my tank (16 1/4) so I ordered a sending unit that was adjustable for varying size tanks.
If you have any other questions, let me know. For someone that has replace several of these, I'd guess its about a 1/2 hr job.
Try to do it yourself. If you can't take it to the mechanic. Save yourself $500