All your buoyancy comes from non rigid devices (vest, suit). As you go deeper those devices compress and you lose buoyancy causing a feedback loop. Compounded with loss of vertical references, you can quickly sink to the depth where the air you breathe will become toxic. At that point you don’t have the mental capacity to save yourself.

More likely that the target depth is down near 50m, which is well into the narcosis range for surface gas mixes. Particularly for inexperienced divers it is very easy to not realize what is happening and become too stupid to save yourself.

Higher salinity may exacerbate that but not by too much.

> become too stupid to save yourself.

You probably weren't too smart to begin with to go to 50m on compressed air, and without a seafloor at that.

I haven't been to the blue hole, but I can easily see nonchalant guides leading people to ignore any misgivings they had from what little training they may have had years ago.

I can see someone with a 30m certification going to 40-45m. But anything below that ought to be a huge "no" for anyone who's done the basic deep dive training, in fact you should be running away as fast as you can.

As a diver descends, they have to add air to their BCD (Buoyancy Control Device) to maintain neutral buoyancy (and release air when they ascend).

Without that compensation step, as a diver descends their buoyancy becomes increasingly negative.

Add a level of narcosis for descending below 40m, mix in some excitement/stress, and some equipment configuration issues (e.g. too much weight), and the task-loading can cause some people to forget/skip routine techniques such as adding air to maintain neutral buoyancy.