All outboard motors have vented gas tanks. As the fuel pump sucks fuel out of the tank, the fuel's departure creates a small vacuum inside the tank. If there was no way to fill that vacuum, it might overcome the pressure of the fuel pump and your motor would starve for want of gas. Some outboard gas tanks have a vent that opens as pressure inside the fuel tank drops, some have a simple check valve, and others have a manual vent you open yourself.
Put your ear close to the tank before you fill it. Listen as you loosen the gas cap to check whether air rushes into the tank. This indicates a problem with the vent.
Open the manual vent valve, if the tank does not have an automatic vent or a one-way air check valve installed, before you remove the cap to fill the tank. Close the manual vent after you have finished filling the tank.
Open the vent after you connect the fuel hose and tank to your motor. If you do not open the vent, your motor will experience a vacuum lock.
Remove the filler cap from the fuel tank if your motor begins to "gasp" while you are making way in your boat. If the motor runs when you remove the filler cap, but seems starved for fuel while the cap is in place, replace the tank vent system.