Blocked sinks

This may sound stupid but I have a blocked galley sink and blocked shower drain. Now I know they drain under the waterline so do they have some kind of non return valve fitted or can I put a plumbers corkscrew down it?
All other solutions welcome !!

I have no idea about showers, but my Galley sink has a seacock in line to the outlet, which is located on the waterline about midships. Mine is a mark 2 and frankly all of them are a bit different.

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