2000 Larson 254 5.7l mercruiser engine bogging down..

[21seapro]

okay guys you're goes.I have a 2000 Larson cabrio boat 254. After 20 minutes of running the boat will just bogged down like it when a ground. I drained out all the fuel from my fuel tank due to having water in tank. I took the boat out again with fresh fuel and still had the problem. I've found out when I release my fuel water separator filter I hear air being sucked in from the fuel tank. When i put the fuel filter back on the boat runs great again for about 5 minutes. Then I gotta repeat the process. I cleaned out the one vent that I could find for the fuel tank and it was clear. I took the boat out again and still have the problem.I even added on a second fuel water separator.

when I disconnect the vent from the fuel tank and a blow into it on the tube side I get air coming out of the tank side where I disconnected the vent tube from. The vent tube goes to my fuel filler cap. Is that normal? If anybody has any ideas I'm at the point I will try anything.