When the "voices" in Lavenza's head urge her to seek out The Million-Headed Serpent, this is the location which they guide her to. It was originally believed to be nothing but a legend, but all legends have some truth to them. The entire place is completely devoid of all life. No reason has yet been found for this. It's more than likely got something to do with the anomalous nature of the place.
Although it is described as a massive body of water that is seemingly infinite in size, it's not an official ocean, and it's supposed location is not known.
It is possible for all living things to breathe under the ocean's waters. Much like almost everything else about the ocean, no reason is currently known for this.