Ive been trying to break the water system and discovered a couple of things… First of all air pressure is clearly not a thing at all. An enclosed space will still be able to fill with water to the height of the original even if the air would be trapped inside.
Secondly slightly more interesting bug is if a wall is built surrounding some of the water in a lake any water tapped from it will act as though it comes from the smaller internal source but graphically the water will still appear at the level of the lake and will not alter. Like in these images.
Third bug, After allowing the water level to settle, if I then mine out the floor in one container then residue water will be left on the squares that should now be empty. See the stairs in the corner of the first image that water wont disappear.
I know that water is still being worked on so I hope that these points will be addressed soon!