As stated, check if you have an anti-siphon valve. It is a fitting with threads on one side and barbed on the other. Threads screw into your pickup tube and hose mounts on the barb. I chased your exact symptom years ago for a long time. I finally talked to a friend of a friend mechanic and he diagnosed it over the phone. It looks like a normal barb fitting but the hex part is usually a little longer and the fitting is usually made of aluminum instead of brass. Also yo should be able to rule out vent by loosening or removing your gas filler cap and running the boat over half throttle. If bulb still collapses then it is not your vent. Good luck!!