Back in the day some sinks in the toilet compartment would be connected to the same seacock as the toilet intake. The sink drain would also have a shut-off valve, so one had a choice in what would happen. Pumping some soapy fresh water
from the sink through the head
was thought to be a good thing. Closing the valve was thought to also be a good thing back in the days when the head
was towards the bow and pounding into a head sea could sometimes force water
past a closed seacock, then one had a second valve stopping the water from getting into the interior
of the boat. Closing the sink drain valve meant that one could pump sea water through the head without worrying about sucking air from the sink.
I think the original poster needs to move that seacock. It doesn't matter if it is plastic or metal, if it needs to be closed and is really hard to get to, you have a serious problem. IMHO all seacocks should be instantly accessable.