It's possible this might be considered in developments in the future if there is more demand for it, but in the meantime you can very easily convert any of the bitmap exports from CC3 to WEBP format in GIMP, which is free software you can get here.

Open the bitmap in GIMP and then go to Export as... in the File menu, and pick your export file type from the dropdown Select File Type, name it, and away you go.

It might not work perfectly, since the BACKGROUND TEXTURE is below the floors, but if you don't need that sheet, or only use it to outline the deepest level of drain, then you won't get too many problems.

Sinister Sewers doesn't use the usual wall cutting tools, so I think you would be all right doing that - as long as you realise it would become quite difficult to separate entities by layer. You would have to separate them by sheet instead.

However, to separate the levels you might wish to have separate sets of sheets instead of layers, so you can cast a shadow (for example) on the lower levels from the upper ones. Layers don't give you that power.

There's a very subtle difference. You can see the edge of the water a little more clearly.

How about just a plain old black polygon with one EFI set up like this?

Or even that plus the Glow Blur Radius doubled to 0.5?

Alternatively, how about importing the water texture from another dungeon style?

I'm not sure why TRACED didn't work for you in this particular instance, though I suspect that if it had worked it might have traced around each of the lines and not the whole shape.

Providing there are still only those 4 lines there and nothing else, have you tried right clicking the Fractalize button and using Line To Path on all 4 lines at the same time?