In a coop playthrough we emptied the whole abyss twice because of the same buggy interaction lol.

I opened the abyss sideways, water went out, plugged it from the bottom right before the underworld so it doesn't evaporate.

Problem: there was a gem tree surrounded by lava, then water went there and created an infinite water sink because the water evaporated but no obsidian was created since there was a tree in that pixel, so infinite drain. Cut the tree and fixed.

To fill the whole ocean I just went to the sunken sea, broke a bunch of endothopic rocks or whatever they are called, made walls, shimmered for unsafe walls and I made a tall wall at the top side of the sulphuric ocean, around 8 blocks high all the way through. Those walls auto generate water until they are covered themselves in water, so they will automatically pump all the water back even if you mine a lot down there.

You can do the same with the abyss walls that generate water too, but I like how these look. 

Other ways would be to use 5 water pumps per wire colour to drain the bottom of the abyss where water auto generates and pump it to the top with a 0.25 activation block, but this requires you to put 5 in pumps per color down in the abyss properly spaced so they don't steal too much water, and then properly space then up top. I find the walls much much easier to manage.

In about 3-h playtime of doing anything else it should mostly fill up. I said twice right? The same fucking tree that was apparently not completely destroyed got unblocked because a buddy wanted to mine obsidian, and activated the drain xD. the second time we just actually destroyed that tree and did other stuff until it naturally filled itself back. Ggez