The water-table system sounds deeper than a typical management mechanic. Are you considering a clearer visual indicator when a mill or ditch is affecting nearby ground, so new players can understand why an area is still wet?
Right now the game only shows it cell by cell. Point at any square and the GROUND panel reads its level, water, salt and how far it has sunk, and on a mill it adds the level the mill holds and whether it's working or stood. What it doesn't draw is the reach, meaning which cells a mill or ditch is actually draining at that moment. That's the missing piece you're describing, and it's a good one, so it's on the list.