The fuel gauge on my J30 hull #333 shows full. It isn't. Before I unscrew and pry it off the top of the tank, can I expect it to be useless or the sender just stuck against the side or the tank or something. I'm thinking it is a simple mechanical device, or hoping it is. Ideas?

It is mechanical. I replaced it a few years ago. It surely is non-linear in it's read out. I use five hours per gallon to estimate my fuel usage off the engine hour meter.