I had that same problem on a 94 Tracker Pro Deep V 17. There is a small plastic piece that screwed into the bottom of the cap on the tank. I think it has something to do with the venting since there is no other vent on the tank. Anyway that little piece came loose and dropped into the fill hose of the tank. I could only add gas by going really slow. This was like this for a few years. I could never figure out what was blocking that fill line. One day I had the boat out in the drive and for probably the 50th time I was trying to figure out the problem and when I opened that fill cap the sun was shinning just right and I could see "something" blocking the fuel passage going into the tank.
If everything was working on the fill tube prior I would look inside the tube. If you have a vented cap it may have come apart and be blocking the fill tube.