So that sub is the length of a person and/or child. But say if there's a bend in the tight spaces of the cave then it won't be able to turn the corner.
Exactly, it seems like a much poorer option to go with (or even a viable option in this case), at least if you have the kids in the water they can manoeuvre themselves through tight spaces of which we know there are some, you need the flexibility to make these dives.
I'm sure if the spaces were larger it may work but the only people it would really benefit would be the rescue divers in case of the person being rescued panicked and put everyone at risk. I would think climbing into a tube like that would instantly panic most people though, they'd then have no chance of survival if anything went wrong with one of the divers carrying it.