Probably isn't fuel pressure and just read back thru everything again, It runs fine with the seat open and does not when its closed, it has to be the motor is not getting enough air, or there are other fumes building up and the motor cannot run.
As an example: Buddy just filled his house boat up and turned on the blower. Started the motor and pulled away from the dock. Got just a bit away and the motor starts choking down and will hardly run. He is getting into trouble and I go back and lift the engine hatch and motor revs up and takes off. Well the engine room was full of gas fumes, and we were lucky it didn't blow up. So if your getting exhaust or gas fumes building up, or if the motor is just not getting enough air this can happen.
So look close at your vents and blower hoses. Don't want to have a bird or something build a nest in them. If all that checks out, maybe add another vent. Adding another vent should not be necessary because the boat was doing fine before all this started.