I guess it depends on what you plan on keeping in it and what looks nice to you. Im currently turning a zoomed into a rock wall palu and the water area is only going to be about 3" deep. Do you reside in atl by the way?
Yeah, it's ALL dependant upon what you wish to keep, as well as the tank size - not to mention your design intentions.
If you intend to have fish in the water portion, then you must first decide what you want and base the design on the needs of the fish. A paludarium involving, say, goldfish would need several tens of gallons of water at a bare minimum at several inches deep, whereas a paludarium involving Axelrod's Rasboras could need just a handful of gallons and just a few inches deep. If you want a paludarium for a turtle, you'll have different needs than a paludarium for a newt.
There is no "you need X inches of water to make it pretty" rule.... A paludarium can be gorgeous and have just a few gallons of water at a few inches deep, or a few hundred gallons at a few feet deep. The tank size, the animals' requirements, your design needs/wants, and your personal aesthetics are all important factors (with the animals' needs being first and foremost).