Interesting article on skindiving (or freediving)....which is really a synopsis of this book:
"There is a point about 30 feet below the surface that freedivers know as “neutral buoyancy.”
Beneath it, the ocean stops trying to spit you out and begins sucking you in.
That is how “Deep” itself unfolds.
(from the nytimes review)
The rest of the NYTimes review:here .