Actually there's nothing dumb about the scene, except that there would be an upward pressure on the boat (buoyancy).

If that was overcome (e.g. with some weights or strings holding it down), it would be perfectly fine to breath in there for a while.

So, aside from the buoyancy issue, not only it's not "dumb", but that's exactly how old "diving bells" worked.

https://en.wikipedia.org/wiki/Diving_bell