Do you have any pics of how the filler hose goes into the tank? Sounds like you should be siphoning the gas into the tank.
If you can get a 1” inside diameter hose to fit in the filler neck and the other end in the gas can, make a seal with tape and some foam, at the gas can about 12” from the end. Just enough so the hose reaches the bottom of the can. Then shove the other end into the boat gas tank, you should be able to push 18”-24” of hose in the boat. Then push the taped foam tight to the gas can and squeeze the gas can. This will push the gas out of the can directly into the gas tank with no chance of getting backed up.