
Goblin Witchlord |

So, let's imagine a small party of heavily armored adventurers walking along a sea floor, passing between them a bottle of air. Well and good! But if they don't reach the surface before the end of the day, they'll drown in their sleep.
Can they use tiny hut to create a shelter to sleep in as they continue their underwater journey? Does it have air in it? Can they fill it with air?
I assume a secure shelter would flood. A magnificent mansion would be safe for air-breathers underwater, since outside conditions do not affect the inside, but would the space created by a rope trick flood, since objects can freely pass through the uncloseable portal?

Ramarren |

... would the space created by a rope trick flood, since objects can freely pass through the uncloseable portal?
Note that I'm ignoring the problems of pressure here, since either it's not a factor or the adventurers have some way of dealing with it as they are walking on the bottom anyway
The good news is that the space in the Rope Trick won't flood, since simple air pressure should keep the water out.
The bad news is that a Bottle of Air isn't going to do them much good in the Rope Trick, since it works "When taken to any airless environment" (Hrm...no good in poisonous air either, probably not was was intended, but that's RAW). So, unless they are sticking their heads regularly into the water so they can breathe from the bottle, the air in the rope trick will quickly become unbreathable
If the party has the skills and materials to jury-rig a frame that will keep the bottle underwater, oriented correctly, and directly underneath the Rope Trick opening, they can uncork it and have a constant stream of fresh bubbles coming in to the extra-dimensional space. Whether it is worth being concerned about air pressure, surface tension, or a million other little issues is solidly in the realm of a GM's call (Though a wizard willing to give up a night's sleep to constantly cast Prestidigitation to make little breezes for the purpose of air exchange might take care of it.)
Using this method is going to be very noticable to anything in the area...

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Object lessons here.
1. Sometimes it's better to push if the surface can be reached in a few more hours. Exhaustion is better than drowning.
2. Always plan ahead on as much as possible when doing underwater excursions and add some wiggle room for unexpected difficulties in your traverse.
3. Never ever if possible depend on a setup like this. If sahauguin being the clever folks they are observe your party and see how you are managing your breathing, they're going to be smart and yank away the guy with the air bottle. sub motto... don't try to be Cousteau on the cheap. at the very least... keep potions of water breathing on standby. Or get your casters to prep the water breathing spell.

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I see nothing in the description of Tiny Hut that it would generate any breathable air, or even be any defense against cases and what not. Sure, the gases might be more moderately tempered. It certainly would flood (well actually I think it would appear already full of water, but same difference).
A Secure Shelter would have all of the same problems, for all of the same reasons.
A Magnificent Mansion would absolutely work.
Rope Trick is much more fuzzy in how it would work under such conditions. I would rule it as being a viable solution, very similar in effect to Magnificent Mansion (but considerably more "ghetto"). Water wouldn't enter it, and air is created in the extra dimensional space for the duration of the spell. However none of that is directly stated in the spell effect, so I would absolutely see another DM ruling that the air comes from outside, and thus it would both flood and provide no actual breathable air to begin with.

Goblin Witchlord |

It's good to re-read the item descriptions!
I had always envisioned the bottle of air as bubbling and constantly emitting air, in the manner of an Eversmoking Bottle or Decanter of Endless Water. It sounds more fun that way, but I guess "unflooding" a sahuagin outpost with a bottle of air and some sovereign glue would be a cheesy way to deal with them.
I'm thinking about borrowing some underwater rules from Stormwrack, or possibly Necromancer's Dead Man's Chest. Pressure is only a serious problem below 100 ft, although temperature could be a serious problem with which tiny hut would certainly help. There's a description of a diving bell there, in which an open diving bell is flooded neck-deep at a depth of 100 ft.
Water breathing is clearly the solution for this problem, but it can take a lot of spell power to keep a party going this way, and it's a good idea to have a backup plan. In case the sahuagin kill the priest, say.

Goblin Witchlord |

Tiny hut is a seriously weird spell.
It's a 40-foot-diameter hemisphere, which is far from tiny. However, only ten Medium creatures can fit into this space for some reason.
It creates a force field that stops rain and sandstorms, but doesn't prevent missiles, weapons, or creatures from passing through.
What happens if a regiment of orcs tries to charge into the durn thing?

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Tiny hut is a seriously weird spell.
It's a 40-foot-diameter hemisphere, which is far from tiny. However, only ten Medium creatures can fit into this space for some reason.
It creates a force field that stops rain and sandstorms, but doesn't prevent missiles, weapons, or creatures from passing through.
What happens if a regiment of orcs tries to charge into the durn thing?
You have to account for the curve of the sphere. There's only so much of the radius that a Medium creature will fit without being stooped over. Do a little geometry and you'll see how much that is. By the way 10 creatures is assuming 10 5x5 foot spaces with enough height. If you want to squeeze people into spaces that's a different thing altogether.
Also... there's nothing that will stop a regiment from charging in and through... but if it's raining outside, most of them are going to stay wet. Think of the sphere not as a physical shelter but as the area of effect of a Protection from Weather spell.