I'm in a naval campaign where some of the game could take place underwater, and I could reasonably acquire a Bag of Holding soon, so it would be nice to know before I try to grab an item underwater and jettison all of my gear into the Astral plane. Would it fill with water until the bag was filled to capacity and scatter all the contents inside as well?
Also assuming you didn't break the Bag of Holding by overfilling it to capacity, would putting water inside it get other objects inside wet, potentially ruining them?
Best Answer
It would quickly* rupture.
Five-hundred pounds of water is only about 8 cubic feet in volume, so the bag's weight capacity would be overloaded after merely an eighth of its volume capacity was reached — assuming it started empty, as any other items in it would help it reach its weight limit earlier. It would then rupture, scattering its (damp or soaked) contents across the Astral sea.
An objection might be raised: doesn't the bag only store things specifically put into it? No, the owner is not so in control. There's no verbiage limiting access to only intentionally stored items, so an open bag allows anything to pass into it. This means that opening it underwater would result in the water pouring in.
The space inside the bag is described as a continuous finite volume, not a set of pocket-spaces for each individual item put in it; therefore, putting/letting water into it would get water on any other items sitting inside.
* How quickly depends on the local water pressure, but just “quickly” is plenty of precision for our purposes.