Minecraft

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.

Update*

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.

Putting the difficulty at Peaceful insures you cannot die underwater. Unlocked for me that way.

So First off this achievement has to be done in ONE sitting, it is not cumulative.
I'm gonna assume everyone knows how to play Minecraft by this point so i'm gonna skip some of the basics.

You're going to need the following....
1. Potion Stand
3. Glass bottles filled with water
1. Blaze powder
1. Nether wart
1.Puffer fish
1. redstone

so you're going to be adding the nether wart and puffer fish together to make a Water Breathing potion (3:00)
Then you will add the red stone to the brew to make Water Breathing Potion (8:00)

With 3 Water breathing potions that will last 24 minutes. A Day/Night cycle lasts 20 minutes so as long as you drink one potion after the previous has expired you will be golden.
If anyone has any questions fire away in the comments below.

*Credits for this solution goes to Paper Sharks, he basically told me how to get the achievement*

If you swim just underneath the surface of the water you will not run out of air.

Swim around for about 20 minutes without breaking the surface. Aim just above the horizon and you should stay at the same depth, but if you drop down too far and air starts to disappear just tap to move yourself up a little - this is safer than aiming higher as you might break the surface and have to start again.

Edit:
Samurai Ralan says:
To add to this: you can breath from a thin layer of air trapped underneath frozen water. That might help keep height in check easier.

TangerineGamer says:
if you find a place where the water is only a block deep, like at the shore.
Double tap forward to swim horizontally and aim for a corner that is only one block deep, then you just need to hold or tape the control stick forward into the bottom of that corner.
You will be in the breathing spot and still under the surface so just need to wait there for 20 mins

Sleep with the fishes (30)
Spend a day underwater.

EDIT: Bubbles only appear on v 1.5.0.4

This is a solution to stay underwater without having to make any potions. Use the seed provided and swim to where I do in the video. The bubble columns from the lava squares at the ocean floor will replenish your oxygen when you enter them. However, standing on top of the lava will hurt you and eventually kill you. The video will show you where to stand so you can breathe indefinitely and walk away from your controller. The achievement will pop after a full in-game day, ~20 minutes.

You will also unlock this if you haven't already:


Seed: -1618472320 (Credit to Dwaggienite)

Play on Survival and Peaceful


Note: You may not see bubbles when you first spawn. Swimming around and/or respawning should get them to appear.

Hope this was helpful

Another way to get this without potions is using a conduit. To use one, (assuming you have and know how to get one) make a hollow 5x5 frame intersecting another 5x5 frame out of any solid prismarine block . Place a block on the center where there's the cross itself and put the conduit on top. Break the block below the conduit and it should be right in the center of each frame. If done correctly it will begin to float and activate. This unlocks moskstraumen achievement. Now just wait until the achievement pops. You don't have to worry about breathing any more and you mine faster.

I don't know why all these other solutions make it so complicated. Just go into the Nether and grab a magma block. Place it under the water and place a block right next to it. Now position yourself on top of the block so that your air gets refilled but you don't get pushed down onto the magma block and take damage. Just wait a day in game and job done.

Step by Step guide on how to stay underwater without the needing to use Potions, or bubble columns.

This glitch should make it easy to obtain a ton of rare Minecraft Achievements where you need to be underwater.

So I just played on peaceful, and had respirtaion 3 on a helmet and I could basically stay underwater indefinitely.

My solution to get this acheviement is by making a bubble elevator ( by use and single column of water soul sand and kelp, so u place your kelp on some dirt and stack it up untill it hit the top water words block then go to the entrance at the bottom of the elevator and break the kelp now replace the bottom block and replace it with the soul sand. Now you have the elevator that goes up.) now that your have the elevator place a block above the top water words block.
note: do not place in the water.
Now water into the elevator and wait for a whole day to pass.
You also will get the free driver achievement after 2 minutes.

( YOU WONT NEED ANYTHING )

Make it Peaceful

stay underwater for a day you wont die

you will gain more health than getting damage lol

Enjoy...

You do not need potions or a conduit for this one.
all you gotta do is
difficulty: peaceful
Seed: ocean monument ahead or coastal village
Now theres this glitch where you only lose one heart and then it goes back to full health just stay underwater for at least 20 minutes.

The achievement unlocked for me.
Good luck

When you go to some of the preset seeds go to Coastal Village. When you spawn you should see a ruined portal. Go to it, there should be a magma block there. If you need an iron pickaxe the village next to it should have some iron in it. After grabbing the magma block go to the ocean, place it down, and hold B to crouch on it for about 20 minutes.