Your tank is vented to the outside of the boat via a (usually 5/8") hose and fitting on the outside of the hull. That's what you need to check first. Fuel can't go in unless the air in the tank can be pushed out the vent line. Some boats have a combined fuel fill/vent in the same deck fitting, but there are still two hoses going to the tank- one fill and one vent, and the same caveat applies. Figure out why your tank isn't venting properly.
My .02
