Have you ever been inside mini ponds. I have been going to them from my childhood since I had them at my native place. Now there is not enough space there to learn to swim but there was depth enough to submerge yourself in it. So I learnt how to breathe in water far before I learnt to swim.
When swimming one needs to be comfortable with breathing in water. You may have become a veteran so you don't understand that phase I believe. But take one if your friends who is not comfortable breathing in water to swimming. The biggest difficulty and fear for them will be that they can't breathe in water. The moment they submerge their face even for a second in water panic strikes.
In the first swimming lesson, by the end of it they are able to swim. If they had breathing issues in water they would never have been able to adapt to it so quickly, I hope you agree with this atleast. This only serves as evidence to me that they are able to do a decent job at holding their breaths. Plus I we don't know how much Sam practiced swimming in the past 6 days of the game.
One thing we do know is that on Day 17 he and Dylan were both good enough to take numerous laps across the pool. Which I don't think anyone with underwater breathing issues will be able to do. You didn't raise any questions back then about this. So I don't understand suddenly it becoming unacceptable for you.
The time period for which Sam was underwater was around a minute which isn't much. And it seems to me like you don't know but vocalists have to train their breath control because there are always songs in which breathing gaps are far in between, So if you would have to be hitting a high note then your voice will fall flat if you run out of breath. So Vocalists have good lung capacity.
And just you don't say that I am saying whatever comes to my mind. Here you go. This is just one of the 100's of results that you will find on this topic which support my argument.
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Also I would also like to address the visibility inside the tub. I came across a render which can give us a good idea of how things look inside the tub from an outside perspective
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From here we get to see the level of visibility inside the tub. We can hardly make out Sophia's legs even though she is standing. This shots angle is a lot higher than Liam's eye level. So what Liam sees is even less than we are seeing in this render. Because of a lower angle. Plus in this render Sophia is far away from the seating space. The closer we get to the seating space the more of a blind spot that becomes. I am not believing in something unbelievable, you just don't want to believe even though there is a good chance of it actually happening.