Digimon Xros Wars Episode Four!

[Guilmon and a shotgun]

Let's not bring that game into discussion. And It seems Shibumi told them they could breath so that's why.

Actually they don't meet Shibumi until AFTER they learn they can breathe the water...Takato uses the Com unit given to them by Yamaki to contact their parents and Henry uses it to ask if it will work.