A quick and efficient way of replenishing your air bubbles underwater is by placing a fence, not many have thought about it, but you can move up really close to the fence underwater, as it has just enough air surrounding it, to give you that one breath to save your life, depending on how deep you are ! -Hope this helps, i will try to post pictures soon, and please leave feedback on any other tutorials you would like to see.
There are a few other things as well. I believe you can still place sugarcane and do it if the ground is dirt/sand. Torches work if there is two high walls, but they quickly disappear so be careful. Glass panes and iron bars. Doors and sponges, like Kuke said... A lot of things. Just gotta think about it.
Sign's also work, but take a long time to break and place so its not recommended Btw, if you are really planning to build something underwater, i would just buy a respiration and aqua infinity helmet.
Theese are just things that makes Minecraft very unrealistic, also the fact that you could place a Ladder or sign, and the water would stop "streaming". However, i didn't know of the fence thing there, so thanks for the share
