Land access height should not start below sim water level

My friend and I built a house on a hillside plot that slopes down into a river. When we turned on land access controls, we noticed it was still possible for people not on the list to stand on our deck, because even though it's right on the water, the ground is more than 20 meters down.

For any point within a parcel where the terrain is underwater, you should get protection from zero to the sim water height for free, and 20 meters above the water, just as if the terrain was exactly at the sim water height.