Unfortunately, that is not very true. It's easier to be a diver, than to be a swimmer. With an oxygen tank, all you need to do is to sink in.
Of course, it's not as easy as I said too, but it's the same idea as going downhill compared to uphill.
So, what happens when someone that always swim upstream, somehow lost it all. And started diving. It's not something easy to adjust, but it's just easier to sink. And the further down you sink, the lower you get to. What if, at one point, you realized, you are stuck down there. You tried to swim up, but, you just can't.
Struggle all you may, scream if you can, but I think you are stuck at a point of no return. It feels like swimming back to the surface is impossible. You don't see any lights above your head.
I can only say, good luck... Better swim up fast, else, I'm sure you'll be killed by the pressure.
