Yea, I have a proper seacock on the sink drain in the head for this reason, but was shocked to see the shower sump didn't have one, even though the exit is at or slightly above the waterline depending on list. I have also heard that anti-syphon valves are good to have until they fail open, and act as a leak source, so I guess it depends on preference. I might have to add a anti-syphon and a shut off of some sort.
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1980 Pacific Seacraft Orion 27
Sailing Grounds: Southern California