Well Max,There are a couple of things this could be but by what you are saying about the bulb eliminates some things. Since it's not going flat it pretty much rules out a venting or fuel restriction issue up stream in the boat. What it may point to is air entering the fuel system somewhere and invading the fuel pump and losing prime. I would be checking all the hose clamps in the system from the tank to engine. Also a bad o ring in the fuel connecter at the engine can cause this, they will usually leak gas if bad but not always. I have also found that in line fuel filters do not completely fill with gasoline when running and can be misleading. I always advocate running the engine on separate 6 gallon tank and fuel line for testing and see if it changes the symptom and work backwards from there. You can also temporarily replace the section of fuel line in the engine from the fuel connector to the fuel pump with a clear fuel line and physically watch the air entering the system, then switch between the 6 gallon tank and the boats fuel system and see the response. You'll have to run the engine with the cowling off with someone else driving while you do this. Just some ideas help to get this pointed towards a solution.