I managed to generate my own map, totally empty. I thought I had done something pretty awesome, until I realized that you can't build onto the outermost surface.
Notice how the water never falls inwards. I think this is because the animation process doesn't happen until you delete a block. So if there was no block there to start with, the animation never starts. I wanted to build my way up there to add/remove a block to test this theory, to see if it'd flood the whole thing, but I had no blocks to attach to. ^^;
This is the standard "small" 128x128x64 map size btw.
UPDATE: I generated the stage to add a lower level of blocks, so now it can be build onto! Be careful of flooding the area though, cause it was so much water that it crashed on me originally. On a server, I could come back, but in singleplayer, it'd prolly just be fucked.
Go here to try it in singleplayer mode. Tell it to load any game, they all load the same.
Now I need to try this on one of the huge map levels.