Not the best way to check the vent
The pumpout hose comes from the bottom of the tank...air in the tank is above the surface. So all you accomplished by removing the pumpout cap was finding out whether the tank was pressurized. If you removed it immediately AFTER flushing and there was NO escape of air, your vent is prob'ly open...but But that doesn't mean the vent can't be partially blocked...allowing just enough air to pass to relieve the pressure in the tank between flushes. So why not do what you should have done in the first place: check the vent thru-hull. That's something you should do regularly and often anyway. Once you're sure it's not the vent, take a look at the inside of the toilet discharge hose. it's not uncommon at all for sea water calcium carbonates to build up in the hose, reducing the diameter enough to restrict the flow of bowl contents--or even completely block the hose.
