There is no fuel pump on the tank like in a newer vehicle. This is a hybrid setup with eVap on the boat, and fuel pump (VST) on the engine. It sound like something is inside the fuel tank itself and floating around until it gets sucked against the pickup tube. Removing the fuel sending unit (round 5/16” bolt/screws) will give you a small port to see inside the tank.
Back in the first several years of eVap boats (2013 est) the, eVap liners were either opaque or black and you won’t be able to easily see much. On newer boats the eVap liners are clear and allow light through the tanks walls.
in any case, we think that’s your issue. Often tanks it’s a bottle cap foil piece or a plastic wrapping on a bottle head. And we’ve seen the 5 to 10 years old grandchild playing in the boat, in the garage, trick more times than we can ever recall. Pop the cap and drop in everything from bubble gum to candy wrappers
BCB