I like to imagine that this isn’t actually water since water actually hurts inklings an OCTOlings I like to imagine they found a way to make fake water. That’s perfectly safe for them.
Man, I think about the fact the inklings and octolings can't swim a bit. I hope the in the future new lore drops that says that a new medical/scientific advancement has mad it possible for them the enjoy things like swimming, going to the beach, rain, etc without bursting. Even if only for periods of time.