Jay........

The piping arrangement is a little more complicated than I describe earlier as there is also an airconcitioner intake plumbed off of the seacock with a block valve between that and the  sink/head.  When at the dock the seacock is open and the valve to the a/c is open but the sink/head valve is closed.  Converserly at sea the a/c valve is closed and the sink/head valve is opened "when needed".   It works ok but I've never left the block valve open at sea for fear of siphioning in sea water (see Bud's note - sort of the same idea)