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.
Another way to achieve this is to build a conduit.
To build the conduit, you must find a “Heart of the sea”.
A Heart of the Sea is obtained from a buried treasure. The location is marked on a buried treasure map, which is found in ocean ruins and shipwrecks.
Once you find one, you’ll need 8 nautilus shells to complete the conduit. These can be found via fishing, Mobs, and trading(I’d advise mobs as it has a higher chance over fishing and trading takes five emeralds, unless you have the hero of the village buff.)
To activate, a conduit needs to be in the center of a 3×3×3 volume of water (source blocks, flowing water, and/or waterlogged blocks), which itself must be enclosed within an activation frame. The frame is built out of blocks in three 5×5 open squares centered on the conduit, one around each axis. Only prismarine, dark prismarine, prismarine bricks, and sea lantern blocks in the frame contribute to activation. A minimum of 16 blocks are required and produce an effective range of 32 blocks.
Here is a link to the page about the conduit on the Minecraft wiki if you want a more in-depth look. https://minecraft.fandom.com/wiki/Conduit
All you do then is sit and wait for it to pop. Hopes this helps! Any questions? Comment below.