Thanks for the replies guys. Both of you expressed the main danger being that the air would be dirty (the other danger being the possibility of what I call "pilot error" - making a mistake on your procedure).
Check out this page about Blue Grotto.
http://underwaterflorida.homestead.com/grotto.html
They call it a "fresh air bell" and specifically say you can breathe in it. I've also read that is has a "bubbler" which I'm guessing keeps that air supply fresh, because if it wasn't continuously replenished, it wouldn't stay clean for long with people "surfacing" in there and breathing.
I'm still not planning on using this anytime soon, but was wondering if it was doable. But I am thinking about doing it sometime, maybe after receiving SCUBA training and some practiced "exhale ascents" over progressively longer distances. I'd also probably want to go with another experienced free diver who's done it. I'm sure there are freedivers that go to Blue Grotto and do this. That's why I was curious if anyone knew this particular spring.
