I'd check the vent hose routing from the tank(if you can see it). You may have a sharp bend, kink in it or possibly an obstruction in the hose itself. Are you reaching the top of the tank before this happens? Where is the vent outlet located? On some boats the vent gets covered by the large plastic flap around the nozzle of the hose.. that can cause problems too. It sounds as though there is a partial obstruction that maybe being pushed closed by the escaping air. Best solution is to pull vent hose off both tank end and vent end... flush with solvent of some sort and blow out with compressed air if you have it.. or alternatively.. just replace the hose and route it so it doesn't kink or get compressed on it's way to the tank. Make sure it's fuel hose rated.. Regular hose will deteriorate quickly and allow fumes to build in places you really don't want them. Another possibilty is that your fuel gauge is faulty... telling you you're not full when in fact you are already... air slowing down means the tank may be full already. To check gauge.. quickly ground sending wire to see if gauge operates properly.. if it does.. you may have a faulty sending unit. Sending units can't be repaired.. have to be replaced.. check with mechanic some of them are integrated with the intank fuel pump if you have fuel injection. The last possiblity is the angle of the boat on the trailer.. if you are tilted wrong the gas tank may not be sitting at the proper angle to recieve the gas at the top of it's capacity. Best way to check is to fill up after you've launched at the dockside marina fuel station.
Hope this helps you out in some way! Best of luck!