I've been playing around with underwater forging, and here's something that came out of it. If anyone wants to do something with it, get it from my fileshare. My gamer tag is PartialSquid.
I did the measurements. The height of usable water is a little over 4 Forge units, assuming you use a flat piece as your bottom base. That means you can have 2 Walls stacked on each other, and you'll still have a little extra. Edit. Alright, I've figured out the delete barrier. It's -9.0. Any object with a height coordinate lower than -9.0 gets deleted. Note, this is based off of the coordinate, not the object's shape. Its body can easily go lower than -9, but if its coordinate goes past it, it vanishes. Time to use those flat pieces, guys.