That shouldn't last long; if water physics calculations get too far behind Terraria will simply perform a "reset" by doing the same settling you see at the very start of loading, and getting it all done in one big, but relatively cheap, burst with nothing drawn.
[doublepost=1501168833,1501168747][/doublepost]
Why don't you just paint water into every single block, even the solid ones? That means when you dig the block up it will contain water filling the space, so you don't have the water level gradually drop either.
(Though remember: only one liquid per block, so you will overwrite lava or honey with this.)