Your seal would have to be too small for a water molecule to fit through. The narrowest distance through which a water molecule could wiggle through a crack is the width of the oxygen atom, which is 1.6 angstroms (1.6 x 10-10 m). So your crack would have to be smaller than that.
Now, realistically, your crack does not have to be that small, because irregularities in the surface of your plate will deform under pressure to create a much better seal than would two perfectly rigid surfaces. This is a complicated chemistry and materials science question, and I'm not the person to ask about it. Generally speaking, however, if there is a pressure pushing the seal together, it will make a water (or air) tight seal. For example, the first Apollo spacecraft were sealed by having a door that opened inward: because of the air pressure inside the spacecraft pushing outward on the door against the metal. This was determined to be a bad idea because it also meant that the door couldn't be opened because of the air pressure pushing against it, and three astronauts were killed in a fire during a test run on the launch pad.
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