The DNA board itself is not particularly sensitive about getting a bit of juice on it; it's the OLED display that tends to fail if it gets wet.
Over-squonking would cause juice to come out of the air hole of the atty. On this mod, that's about as far from the display as you can be.
It's not sealed, although the area most prone to leakage where the bottle is right below the 510 could be caulked to seal it from the electronics after assembly and testing, I suppose. Personally, I don't plan to do that. I do check for leaks every time I change batteries or fill the bottle. In several weeks of use, I've seen nothing significant in the way of leaks.