I dont know how your fuel tank filler hose is routed but if you can check it for kinks or a collapse because of a sharp bend. also check the vent hose, on cars those will be routed to the vapor recovery system. With all this ethanol in the gas now it can cause the hose to decompose enough that some time a flap of rubber will come loose inside a hose. You can blow through the vent hose both ways to check that, but sometimes the filler neck is easily diagnosed with one of those snake like cameras, that way you dont have to tear things apart. If the vent has a check valve make sure it isnt stuck. I'm not a boat mechanic but I have experience with RV's and their long filler necks.