I tested the underwater glitch in L2. I couldn't use the exact combination of skills given in the description because of "Insufficient Memory", however the actual glitch does work!
Basically if a lemming is both a swimmer and a magno booter, he will splash into the water and continue to behave like a regular magno booter. When he walking along the bottom of the water, you can give him another skill and the skill will work normally, since he's technically on the steel underneath the water. If you raise his position even one pixel, though (e.g. using a builder), he will immediately start swimming.
If you try this trick with a lemming who is not a swimmer, he will simply drown when he hits the water.
I've also determined another way to get lemmings walking along the bottom of the water, and that is by crawling up there from below. This doesn't even require swimmers to work, but be warned that once in the water, non-swimmers are pretty much stuck there. Trying to jump, build, or fling them out will result in drowning.