[Sombre]

I don't think you can explain a logical difference between barrel of air and a theoretical 'barrel of water' style item, because barrel of air already defies logic. How does a magic air barrel allow a bunch of fully armoured mounted knights to charge around underwater as if they were on land?

Because it's magic, of course.

Similarly a 'barrel of water' item would work because it's /magic/. Simple as that. It allows underwater stuff to act as if they're in water when they aren't, breathing and moving as normal.

It's an item I'd like to see in the game and unless it was seriously undercosted (not an issue with 90% of items in basegame) it shouldn't mess with balance too much.