The Guide above is a perfect way to get the achievement, however there is another way to get this achievement without Water breathing potions. All you'll need is one solid block and a bubble column.
First, you'll need to go to a Deep Ocean Biome, which in the new update is usually where Elder Guardian Monuments spawn. Once you arrive you'll begin to come across Ravines that have these huge bubble columns sticking out of them, getting in the flow of the bubbles will replenish your oxygen gauge to full.
Once you found the bubble columns, jump inside and this will take you to the bottom of the ravine, this is where you'll place a solid block right next to the column and get to the very edge of the block till you are in the bubble column receiving oxygen.
All you do then is sit and wait for it to pop. Hopes this helps! Any questions? Comment below.