I think you just broke DF, not beat it. Remember: in DF there are no laws of physics. Leave that lake there for several years. And yes, I would like to see that save
DFFD is being quite silly at the moment for me. Chrome isn't uploading anything, and Firefox can't log in. It's quite odd, but I do have a server space of my own and I could put the save and related files there.
You'll notice in his video that he loo'k's over the tiles that are magically suspending the water, and they say Chasm, not Open Space.
Magma flow tiles are sort of pseudo-chasms, in that items and creatures will fall through them to the abyss, yet magma can still rest on them. Maybe something like that is going on.
That is most likely what is going on. The bits of water that fall from the upper z-levels seem to disappear upon touching the upper bit of the new lake. I could potentially test or break this theory in a way. I could remove the doors to the lake at the bottom z-level, or I could dig around it and open a way. I won't do that, however, until I have the save released publicly over the interwebs.
One thing that is bugging me is that the 2 z-level lake has a water color as if you were 1 z-level above the water itself (the darker water color seems to indicate that there's water on the z-level below), but for both z-levels (if that makes sense).
Basically, I think the game is telling me that there's water on a z-level below, but it does that for both z-levels. It is a very, very weird bug.