But a note of caution...
On our 1984 E30+ I used to close all the seacocks, and could not figure out why the galley sink was half full of water (without the drain plug in it) when I returned sometimes. I puzzled over that for a while, until I discovered that the port cockpit drain and the galley sink drain exit through the same thru hull seacock. By closing that seacock, any rain would fill that cockpit scupper hose and eventually back up into the galley sink, which must be about that same height.
I have therefore learned to keep that seacock open, but close all the others for the reasons others have mentioned above. I'm hoping this may be helpful to other owners with the same cockpit/galley sink set-up.
If anyone can explain why Ericson used this strange arrangement, I would be curious to know. Also, if anyone has re-routed their scupper or sink discharge hose to fix this, without adding another seacock, please post.
Thanks,
Frank