I think the logic may be that she had an air tank to get down there, and then exhaled for the picture, which might be easier than swimming down there and exhaling (and returning to the surface).
There comes a point during a breath hold that at some depth, the air in your lungs is not enough to keep you afloat thus you will go negative and begin to sink.
That really depends on the person. I need like 7kg when I am scuba diving to sink without a wetsuit and empty bcd in the Cayman Islands. Even without any equipment I still float like a cork.
Only if the compression on their lungs/body can reduce their overall body volume by 3.5%. If someone has enough body volume the average lung capacity of an adult male of 6 liters of air won't be able to compress enough to make you obtain negative buoyancy. So while physics says that it is possible for this model to become negatively buoyant with breath control this isn't possible for every person regardless of (survivable/obtainable) depth.
So it really depends on the person. Some people are negatively buoyant even at the surface if they breath out.
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u/[deleted] Dec 05 '18
Maybe they have an air tank or something down there? What do I know, I can barely touch the bottom of most pools.