I dont know boats but with my wave runners problems like those are usually corrosion. check the connections on the gauge first,its the one place that never gets cleaned and the salt air can cause a build up.
my jeep sending unit did the same thing. my mechanic said that the ethanol levels in the gas have been screwin them up. not sure if he is right, but he said he has replaced several sending units in the past couple years. i ordered one from jc whitney and put it in a couple months back and within 2 months it was not reading accurately and then just stopped reading all together. maybe just try a new sending unit. i fill my jeep up every 4 or 5 days no matter how much i drive, just so i dont run out, til i get off my butt and order a new sending unit.
I agree with gargoyle. Check the connections at both the sending unit and on the back of the gauge. You may also not be filling the boat all the way up when it's on the trailer, like huntingfool said, especially if the boat is not sitting level like it would at idle in the water. I find it's a little more expensive, but best to fill the boat up while on the water in the marina for a couple reasons. 1. Marina gas normally doesn't have ethanol (I know, some do now), and 2. The boat is setting level, so filling up will definitely fill it up. Just make sure you have a catch can around the overflow vent so you don't dump fuel in the water.
