After nearly a year of exploration, Chinese researchers have determined that the [deep blue “Dragon Hole” in the Paracel Islands, called the “eye” of the South China Sea] sinkhole is likely the world’s deepest, reaching about 987 feet below the surface and surpassing the previous record holder, Dean’s Blue Hole near the Bahamas, by more than 300 feet...I find the latter observation to be the most interesting aspect of this report. I know of no reason why deoxygenated seawater would be heavier, so I presume the anaerobic environment develops because of consumption of oxygen during breakdown of organic matter that falls into the hole, and a lack of circulation of the water within the hole. But I would think there would still be extremophiles down there, and that when they refer to "life" they are referring to oxygen-dependent life.
Researchers told the television station that after about 330 feet, the water is oxygen free and likely unable to support life.
Addendum: Here is a reply I received from a relative of mine who spent his professional career studying similar environments:
Some of these marine sink holes contain water that is more dense than normal seawater due to saltier ocean conditions in the past. If a full scientific report references a "chemocline" that's what is implied. There will certainly be microbial life, almost certainly based on use of sulfate or elemental sulfur in anaerobic respiration of organic matter that ends up in the hole. These are quite "garden variety" bacteria - well known from marine sediments a few meters below the sediment-water interface. They are not really even "extremophiles", just well known anaerobes. Fermentative bacteria would also be part of the microbial food chain.