When i bought my boat, it was 10 yrs old and even then the fuel
sending unit didn't work. Over the years, I have replaced it 4 times
and it has NEVER worked right. Those damned sending units are such
crappy technology...electrical/resistive/ mechanical/floating going to
a meter, and they really expect this to work for any length of time on
a boat? So....
Useless idea #3741 (I am really going to try this one)

Take two pieces of flexible tubing maybe 1/2" diameter filled with
hydraulic fluid connedcted to a T. The base of the T has a 1/4" clear
hose going up to where it is visible in the cockpit. The two pieces
of 1/2" hose support the fuel tank whose weight compresses the fluid
causing it to rise. As the tank empties, the level in the clear
tubing falls. So, it is really measuring weight. But, no
electricity, no moving parts (except the fluid) and absolute
simplicity. Now, the problem, most fuel tanks are fastened down
firmly so when this thing is installed, it must be able to move a tiny
amount up and down (maybe should use 1/8" clear tubing).