Hi my name is Mike,
There are a few things that could be causing this.....
First, to answer you question regarding the primer bulbs..... yes, each primer bulb does cause restriction in the fuel delivery and there should only be one so start by removing the other two and consider running a new fuel line so that you don't have splices in it. The only primer bulb should be near the motor and before the Racor filter.
As for the connections on top of the tank... The overflow and the vent are the same fitting and should lead to an thru-hull vent fitting. If there is a hose that doesn't connect to anything then that needs to be hooked to the vent or capped off.
Another issue here could be the screen on the pick up tube (if it has one) is plugged or the anti-siphon check valve at the fitting on top of the tank has failed. (should allow fuel to pass thru toward engine but not back into tank).
The vacuum operated diaphragm pump on the motor is also a culprit here and have a wear life and are known to cause fuel starvation issuers. They are rebuild-able but I recommend replacing the whole pump.. here is a link to a parts diagram and listing for the pump.
The racor filter is also a water separator and you are right about periodically draining the bowl to check for water...and yes, the clear bowl should always be full.
If the fuel system is set up and working correctly there should be no need for an electric booster pump so try these suggestions first and this should take care of your issue. Let me know if you have any further questions and I will continue to assist you.
I hope this helps.... If you feel that my answer was helpful, please don't forget to use the rating feature as this is we are credited for out time.