In 1.13, we will be able to breath under water by placing Lava blocks & Soul sand which create bubble columns in 2+ block deep water. While this is nonsense in real life (steam is not breathable), it seems like a cool option to have. I am not sure what I will do with it to breathe, but it could be a great addition to the floating items so that we can easily transport things in water downward. Soul sand will push items upwards, I am not sure how this compares to normal water that has items floating.
Since the “air meter” of you character will not recover instantly when you get to the surface, previous ways of working underwater will have to be changed anyhow, so using the lava and soul sand blocks will be useful.