So the vent line is right next to the fueling line. They both run from the gas tank to the fill port on the side of the boat. I took off the gas fill port cap, disconnected the vent line from the tank, and blew some compressed air into it. Air went through just fine.
I did notice that the fueling port has a little black plastic cap inside of it, where the vent line connects. I'm not sure what this little cap does, but it can move in and out (open and closed?) if I pull on it with a bent paperclip.
Next time I go to fill my tank, I'm going to make sure that cap is pulled out and see what happens.