It definitely doesn't. Animal from the deep sea are used to being under higher pressure, that's an entirely different issue and not what I'm talking about.
Fish of any level will be supported in the water in a way that they won't in air so they haven't developed the structure of land animals to keep their organs from crushing each other.
Then there's a whole host of other issues that occur - https://www.researchgate.net/figure/Physiological-responses-occurring-when-fish-are-removed-from-water-Effects-of-air_fig1_281930182#:~:text=exposure%20can%20be%20considered%20acute,(Ferguson%20and%20Tufts%201992).
Even when only removed from the water for a short time a proportion of the fish die after being put back. When the time is increased to do measurements, take photos, whatever else you want to do, the proportion that die shortly after increases quite quickly.
I'm going to say something different from most, this guy might be genuine, he might just be some kid thats a bit lost. He might be a scammer but we haven't heard proof of that yet.
HOWEVER, you can not move a stranger, however lovely, in with your teenage son. That's just not a thing. If he wants to come live with you, AND he can get there by himself, then why can't he live on the sofa? You also mention your siblings being supportive, why can't he share with them? Are they younger than you?
Could he even move there legally? Would there be immigration issues if he tried?
See if you can read the messages yourself between the two of them, try to work out who's suggesting these things. Is he talking about lack of money a lot? Is he constantly having 'situations' that your mom needs to fix?