Well I'm a HE'er (Hollow Earth Theory Believer) rather than a FE'er, but assuming their model of a flat earth is as simple as it seems, the tides could easily be explained by a gentle rocking back and forth motion, a gentle swirl even, of the platelet.
With the hollow earth theory, the moon (and to a lesser degree, the sun) create a drag on the water, pulling it back so to speak, while the general rotation of the hollow earth keeps the water pressed down upon the inner surface upon which we are held against by the same centrifugal, or as some suggest, centripital, force.
Note that when the moon is full, the tides are at their highest and lowest, and that the higher tides are always at night, which just happens to coincide with the full moon being on that same side of the planet right at night (note the moon only 'rises' more or less at sunset when it is full).