Mess table is operable (a dupe can eat on it) when there is a ladder underneath it.
Mess table is operable (a dupe can eat on it) when it is flooded
Build a mess table on a tile, deconstruct tile, build ladder underneath. Put more than 400kg of water to flood a mess table. Dupes can still eat on them.
(...or load the save file, let the dupes eat and observe)