Re: Marine head won't pump
Also sounds like a plugged line to me.
To quickly rule out a plugged vent, you can open the pumpout fitting and try to flush. If it works, the vent is probably plugged. Mine plugged up last in the vent fitting at the tank, not the actual vent line. Also, if the vent is plugged, you'll probably hear pressure (or worse yet, effluent) escape when you open the pumpout fitting, so open carefully.
If it turns out to be a plugged line between the head and tank, you could first try pumping it out, and while pumping, open the sea water intake and pump the flush valve a few times to try and run water through the line. You might get lucky and clear it without disassembly (sort of like a vacuum plunger) .