This looks kinda weird

The diagonals start to fill up from below

Before - a superflat ocean

Directly after doing //cyl 0 100 -100 using worldedit (creating a 100m air cylinder)

That was tested when adding the water draining. The thing is that water is not a static block which stays where it is. so if you make a large circle in the water then the edges would fill up and look very irregular and weird.

They go up and to the center until it is completely filled again.

Filled like before - What we learn is that when creating dry arreas we must avoid creating diagonals in the water edge.

This should also have the benefit of faster calculations since less blocks need to be changed