I've read about decompression sickness and nitrogen narcosis, if it's related to either of those. It's the same issue -- if you don't know what you're doing, it's dangerous. Same with a submarine. Same with a diving suit.

[He sighs.] People end up training for years just to learn how to use those things and condition themselves for the stress. If I had a purely magical way to negate pressure and allow underwater travel, this would be easy. [Too easy, even.]