Cant tell what the water updates will bring but the game did have spawners and despawners as part of the game.
you can read it here covers it more than i can anyways…
but again i dont know what updates will bring or what is in the plans for water for that sake.
And about water air dynamics vs water pressure try doing your google illustration at home in a plastic bottle you can close or stuff with your finger or something
its actually air pressure that makes the effect more than anything else…how you should think of it without a cap on it has 10km of air laying ontop of that water. And as light as air is that’s a lot of air, pushing down on that water since that equalizes air pressure from inside to outside the container.
gets bit more complicated the other way around but ill give that a shot as well >.< but i might mess this up! 
so what happens with a cap on the container is that air pressure will be greater on the outside that on the inside of the container. so much that air will be forced tru the holes in to the bottles since those 10km of air wants to equalize.
when air enters the container it creates overpressure pushing out water again, since overpressure is a stronger force than 1 atmosphere of pressure! Hope i got it all right! or at least it makes a bit of sense 