And particularly, the results of any damage to an air tube -- cold, opaque water rushing in.
Which sounds like a great recipe for panic and death.
And if you're protecting for that (e.g. by walking through the tube with breathing equipment), you're just making the situation more difficult and less like anything anyone has experience with.
1. strong current messing with your construction
2. vertical sections and obstacles that will make your tunnel impossible to crawl through
3. corners, sharp outcrops etc. that might cause damage to it
4. sections that are too long for your construction to be stable even in good circumstances
If you managed to somehow solve all of the above with your design, you're still left with a construction job in an environment where people died just trying to traverse it.
There's like a myriad reasons why this is a bad idea and will much more likely result in awkward failure or even death instead of a success.
Let's not even talk about getting this assembled out of sturdy-enough materials before everyone is dead anyway.
And even if you manage that: One tear in that construction and you can guarantee the death of any child still in it.
If I had to choose I'd go with the professional cave divers and on-scenes experts rather than the dreams of some guy a thousand miles away with questionable, hardly-relevant credentials.