Yea, I'm sorry, I just have to point out. There never has been and never will be a classification called 5th degree burns. The 4th degree for burns are when the skin, flesh, muscles, nerves or bone that this label is applied to is dead and you are going to have to amputate or debride.
What would the fifth degree then actually do, except sound more severe/cool in fanfiction. After your flesh is cocked and dead do you care if it's also carbonised?
Edit: Ok, checked again. There are classification systems (not international standard as far as I can tell) that have 5th and 6th degree burns... they seem to simply divide the 4th degree burns into different depths. If the skin is dead it's 3rd, if the flesh under that is dead it's 4th, if the muscles are dead it's 5th and if the bone is dead it's 6th. Amputation is really the only treatment for anything above 3rd degree so I don't know what these extra levels are for, 4th degree anywhere but on a limb is lethal so...??? Or have I misunderstood?
The amount of abuse the human body can take before it's absolutely, guaranteed to be unrecoverable (forgoing brain or spinal injuries) is ridiculous from a biological standpoint. The primary reason major injuries kill without treatment isn't the injuries themselves, but the complications that arise from them such as infection, sepsis, gangrene, etc.
Medical professional here to weigh in on the subject, and to make that suspension of disbelief a little easier to swallow.
First off, human beings? For as fragile as media likes to depict us being, especially pick-an-action-flick-go-on-I'll-wait, we as a species are quite frankly, stupidly durable in a lot of ways that inspire disbelief when you take the time to think about it. When it comes to violence especially, like a certain parrot once famously said, you'd be surprised what you can live through.
The human body has some quite fascinating mechanisms to facilitate survival in extraordinary circumstances, even without outside intervention. You're bleeding? Not only do you start producing clotting agents, your blood vessels in the affected area will actually contract to further slow blood loss.
Starving to death? Not a problem, because you have fat stores, and if they run out, then your body starts cannibalizing non-essential areas.. which honestly is pretty much anything that's not the brain and spine.
Illness? Your body starts cranking the temperature up. Waaay up, because humans can endure high temperatures far better than most viruses and bacteria.
In my home town of Pittsburgh, there's a fantastic story about a construction worker who was crushed by a bridge. There's a documentary that shows the guy as an old man, doing pretty well for himself despite losing a leg (in perhaps one of the worst ways to lose a limb).
The list goes on, but my point is, the human body readily can and will sacrifice portions of itself to keep the rest alive,
whether we want it to or not. Sometimes, this can be a good thing, though these days, that's typically only a good thing in extraordinary circumstances, such as being buried alive, hit by a train, shot and/or stabbed multiple times, etc. Most other times it's dreadfully incovenient at best or possibly life threatening, such as when you get exposed to peanut butter or pollen or crustaceans and your body suddenly decides "aaaw shit son, who put that fuckin' cyanide-shit there?!" and damn near kills you with it's overreacting bullshit, like an overprotective father trying to pull his gun at his five year old daughter's birthday party because the four-year-old boy two houses down gave her a dandelion.
Now, in the case of horrific fourth and fifth degree burns, possibly multiple compound fractures, hypothermia, multiple infections, a likely concussion, and fuck-all whatever else...
Assuming that Lisa was in the water for less than twenty minutes before being rushed to the ER, I'd be.... Let's say tentatively, cautiously optimistic about her survival, depending on how low her core temperature was. Funny thing about hypothermia. A lot of times, it's dangerous. But when you're already absolutely WRECKED, depending on your age and how healthy you were before getting wrecked, it actually might keep you alive.
Her vitals would've slowed down a lot, but they likely wouldn't have stopped, and a young brain doesn't do too bad at low temperatures if it's not that long. That woulf give a good ER team a lot of leeway, especially if her lungs were still in good shape. She almost certainly would've lost at least one limb, perhaps most of her fingers and toes, a good deal of skin and muscle tissue would have to be removed, and likely some of her intestines, but as long as nothing got in her lungs....
*frowns thoughtfully*
... I'd put money on a patient in Lisa's condition coming out of an induced coma, without hesitation, as long as her lungs were good. Now if she developed pneumonia on the other hand on top of everything else...
*grimaces*
... I'd be perhaps a little less confident in that case...