I have been ambushed by my team.

Thay had a cloth pit it works like the bag of holding when its folded up. (a 20 wide and 20 foot deep)

4 chars got into the pit filled with water and they had the water brething spell cast on them witch means they can brether under water for 24 hrs.
and there 5th char (a druid) pick up the cloth, turn in to a bird and fly off to help them escape.

I thought it was a very good way fo them to get away. then even cast the last of there healing spells in the pit too.

Has this happend to any other DM. and your thoughts on it please :) 

Water breathing does not remove your need for oxygen. It just removes your inability to draw oxygen through water. You won’t drown in the bag of holding (or pit of holding, whatever), but you won’t last any longer than you would otherwise either.

It’s a cool idea until you actually think about it. Different people have different thresholds for suspension of disbelief. Some people can ignore that things don’t actually make sense and enjoy stuff like this. But I can’t, haha. I’d be legitimately upset to play in a game in which the DM let this work. I’d just be screaming “that’s now how any of this works!” XD

It's actually possible for fish to drown. Likewise it would be possible for you to drawn even with waterbreathing.

Drowning just means asphyixiation while in water or similar liquid that prevents respiration.

Fish respirate by having water move over their gills which collect loose oxygen molecules from it. If you seal them in a tank and never resupply the air - eventually there will not be enough loose oxygen in the water, resulting in the fish asphyxiating because it cannot respirate in that water -- it will drown. This is why the top surface of fish bowls must never be covered and why tanks have air pumps.

Waterbreating lets you intake water into your lungs to absorb loose oxygen from the water, much like gills, and then exhale that water. I.E. breathing water (breathing means taking something into lungs and then exhaling out of them). If the water doesn't have enough loose oxygen - you'll still asphyxiate / drown.

Bags of Holding have enough air (i.e. oxygen) for one creature to breathe for 10 minutes. If filled with water, this will likely be reduced significantly to only the oxygen in the water, which will be used up quickly. Filling something with water pushes most of the air out of that container, so the only "breathable" oxygen you can use is whatever was in the water.

So, I'm in the same boat as Saga in that this wouldn't let you be in the bag any more than normal - if anything, it'd be much less.

Some may say "it's fantasy" but while I agree this can explain a lot, having to change "what lets you breathe" is going to cause issues since now you have to rethink how fire and plants work too.

The intent of bags of holding having such limited air supply is so you cannot use them for easy transport of people. Going around the intent by using a low level spell seems odd to me. There are other spells that can aid travel.

Only time my characters use such things for non-storage is when they have a portable hole and decanter of endless water so they can take a bath whenever they want.

Portable Hole has enough air for 10 minutes. I think that would mean your pit would allow 80 minutes of air.

What Wysperra said based on the dimensions you gave. Another note. 

"If filled with water, this will likely be reduced significantly to only the oxygen in the water, which will be used up quickly. Filling something with water pushes most of the air out of that container, so the only "breathable" oxygen you can use is whatever was in the water."

This is not the case. The reason fish can breath in water is because a molecule of oxygen is smaller than a molecule of water so oxygen permeates the water easily at normal atmospheric pressure. Fish are not fission reactors splitting molecules of water to remove the oxygen. Casting water breathing and turning you essentially into a fish would mean that you would be able to breath as you would in a normal bag of holding not filled with water for the same amount of time.

Quote from BTI_Brian >>

What Wysperra said based on the dimensions you gave. Another note. 

"If filled with water, this will likely be reduced significantly to only the oxygen in the water, which will be used up quickly. Filling something with water pushes most of the air out of that container, so the only "breathable" oxygen you can use is whatever was in the water."

This is not the case. The reason fish can breath in water is because a molecule of oxygen is smaller than a molecule of water so oxygen permeates the water easily at normal atmospheric pressure. Fish are not fission reactors splitting molecules of water to remove the oxygen. Casting water breathing and turning you essentially into a fish would mean that you would be able to breath as you would in a normal bag of holding not filled with water for the same amount of time.

 
You have misread my post.

I never said fish gain oxygen by splitting it from water molecules. I said their gills acquire LOOSE OXYGEN MOLECULES (emphasis not shouting since I mentioned this multiple times and somehow you still missed it) out of the water. This is also what our lungs do - we siphon loose oxgen molecules from the air.

Also, air and water have different pressures. As you fill a container with the denser water it will push out the air. This is why when you fill a bottle of water, it's not doubling in size from trying to fit two volumes into the space of one and the water isn't fizzy with air bubbles. So, yes, the bag of holding will end up with less breathable oxygen, as most of those loose oxygen air molecules are pushed out with the air. The only remaining breathable air are the loose oxygen molecules the water has and mainly a few that dissolved into it from the air while being moved out. Generally speaking the amount of loose oxygen molecules is less than in air, because water is denser than air. Just because you now can get oxygen from the medium of water as well as from air doesn't mean your entire bloodwork system has changed to work with less oxygen like most marine life can. The same space of water that could keep a fish of equal size alive for hours now sustains you for minutes - because your body uses more oxygen.

In any case my point was that you'd have time equal to the bag of holding, but more likely less. Neither case means filling the bag of holding makes it more able to transport people for longer  if they had waterbreathing which is what the OP was suggesting.

I didn't see the first 3 sections of your post actually Cyb3r. When I was looking at it it started with Waterbreathing lets you intake... So I apologize about that. Don't know if it was my screen or I was just bugging. 

I still don't agree with some of what you're saying though. Although I have agreed with you on many other posts.

"In a stable body of water with no stratification, dissolved oxygen will remain at 100% air saturation. 100% air saturation means that the water is holding as many dissolved gas molecules as it can in equilibrium. At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere "

Reaching equilibrium does take time but I didn't see anything in the original post saying they had filled the pit with water. It said they entered a filled pit. Maybe that's an incorrect assumption by me. I guess it's possible they filled it with 8000 cubic feet of water somehow in which case, yes there would be less air available until equilibrium was achieved. But saying that Generally speaking the amount of loose oxygen molecules is less than in air, because water is denser than air, isn't correct which was really all I was trying to point out. Water is permeable to the molecules that compose air.

I do think it's a clever idea though. Depending on the game, and my players I would likely allow it just for the creativity. 

For the sake of simplicity I would tell my players it has X amount of air regardless of whether it has water in it or not. Keeps the game moving smoother =)

I remember I got into a discussion about this in a recent game. My character is a Tortle Storm Soul Barbarian, so he's very familiar with sea animals and the logistics of breathing underwater. One character asked if fish can drown and he was able to explain that, yeah, it's possible, if they're placed in a tank or something that can't take in more oxygen.