My mermaids have returned, and now they're dragging semi-conscious people about 700-ish meters underwater. Is it realistic for the person to still be alive at this point? (Y'know, for about ten seconds until my baby girl carves him up and shares him with her colony).
It's blue, Sir
Since most people can't barely hold a minute, and 99% of the population won't exceed a mere five minutes, they're dead. Very very dead. (if you take 10-ish minutes). The duration for reanimation after loss of consciousness and oxygen is around 3 minutes if I remember correctly. So barring the pressure problem necessiting a full analysis (that I can't provide), they're gone (beyond reanimation) from drowning, with that delay.
But maybe your mermaid has little mermaids hungry at home and swim faster. 700m in 10 minutes is barely 4,2 km/h (1,167 m/s). A whale can swim at about 50 km/h
when traveling (13,89 m/s). So unless my quick calculation is massively wrong (it can be, I'm doing that off handedly in a rush), it could dive to that depth in 50 seconds. I suppose the process is a tad more complicated, but you may have a fair percentage of people holding their breath that long.
Bonus : the current World Record for static apnea without assistance of pure O2 is held by the French Stéphane Mifsud at 11min 35s, since the 8th June 2009. The second best recorded was 10m 12s.
Edit: double checked, whale can reach 50km/h during short burst of speed. To take an animal of similar size, the bottleneck dolphin travel at about 11 - 12 km/h. So it would take between 210s (3m30s) et 229s (3m49s) to reach that depth.
Dolphins can apparently swim at 20km/h (126s - 2m06s / most people unconscious/dying), but I'm no marine biologist and this is getting convoluted.
My mermaids can move around 65mph, max.
Well, that's twice as fast as a whale speeding (roughly 105 km/h), so at full speed, your mermaid can reach a depth of 700m in a mere 23 seconds. Even accounting stress and surprise (and given your mermaid is not slowed down by hauling deadweight behind her), most of your "test subjects" will still be conscious - or at least not unconscious from air deprivation.
So you just have to factor the massive and quick change of pressure,
potential thermal shock1... But yeah, potentially, you can still have a oxygenated human in that time-frame.
Also, verified: the 3 minutes duration is the amount of time you have once deprived of oxygen before suffering from irreversible brain damages. You could (theoretically) bring someone back to "consciousness" after a longer delay. But you would most probably bring back a vegetable (effect worsening with time spent without oxygen, obviously).
1: Actually, temperature wouldn't be that bad, it would be around 4° on average. Not what I'd like to dive in, but you could do worse. You'll only suffer from severe hypothermia after a while, so you might be dead from another cause before that.
It's Black, really, really black
There is rarely any significant light beyond 200 meters. NOAA
While it is possible they may be alive (not technically brain dead - yet) they, at the very least, if conscious, are going to be in great pain and probably convulsing.
What happens to the body at 70 bar?
P1V1 = P2V2
V2 = V1P1/P2
Every airspace in the body will be reduced to 1/71st (~1.4%) of it's original size. No eardrums. Need to fart, not any more. Had a fracture that has healed with a void, got an airspace under a tooth or filling, ow, ow, ow. Got a cold? Now you have very small sinuses. You get the idea.
And, as per this link, the ribcage and larynx have probably been crushed. While this is under ice listen to what the guy describes at 60m. You are proposing over an order of magnitude greater.
Nitrogen narcosis, @Alexander has this right.
Oxygen toxicity, O2 becomes toxic at partial pressure > 1 bar. This is what limits recreational SCUBA on air to 40 m (5 bar). Your guy is at 71 bar! CNS toxicity starts first. Lots of factors here but it can set in in < 10 min.
So, agonizing pain, vertigo, convulsions, tunnel vision, asphyxia & vastly reduced cognitive function.
Let's say our protagonist escapes and somehow makes it back toward the surface, shallow water blackout.
And if all that is insufficient let's hope our protagonist does not breathe any saltwater, 25%-40% fatal.
And btw, greater descent speeds make all this worse. More un-metabolized O2 in the system, less time for natural equalization.
So technically alive, possibly. Chance of remaining alive without significant intervention, zip.
I don't believe there is enough real data to rule out the possibility that someone could survive, briefly, at those depths.
It's possible to reach ~500m with special equipment and breathing apparatus. Atmospheric Diving Suit Modern suits are "Capable of operating in up to 2,000 feet (610 m) of seawater."
Freediving is usually less that 70-100m in depth. That would be the maximum, usually it's much less.
If people are semi-conscious to begin with, then it's more likely that they will experience freediving blackout. From the article: "An unconscious diver loses voluntary bodily control, but still has protective reflexes that protect the airway."
So it seems that being pulled underwater is more likely to cause loss of consciousness than instant death.
From https://www.pmel.noaa.gov/eoi/nemo1998/education/pressure.html your poor victim will be affected by Nitrogen narcosis at roughly 30 meters deep causing them to become dizzy and unable to make proper decisions. They will then enter into a stupor before blacking out or going to sleep.... at 30 meters (website says 100 feet).
At 700 meters deep and 70 Atmospheric pressure, your poor victim is essentially dead and would have no way to recover even if he was alive. The concentration of Nitrogen in his blood would be incredibly high and it would feel like he was a stress ball being squished for all he is worth.
Also, your victim isn't equalizing their ear pressure, which will likely result in them bursting their ear drum and being in a lot of pain further disorientating them.