Our group is 5th level: one wizard, one barbarian and and one warlock (old_one/tome). We got one ring of swimming only, and desperately need to find a sunken ship under the moonsea. The barb can swim pretty well, but wiz and lock are helpless without the ring. Lock can see in darkness, so he will be doing most of the searching for the shipwreck.
So we thought of using a leaded canoe upside down for air, and then doing the following
(I'll use the acronym LTH to refer to the spell):
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Wiz dons the ring, then he and barb dive from the shore and carry the canoe. Canoe should be full of air. Use it for breathing sporadically.
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Wiz and barb (plus canoe) reach a reasonable distance under the sea. Wiz pops head and spellbook in the canoe air pocket and casts LTH as a ritual, barb at his side.
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Barb gets the ring, swims back to shore, gives ring to lock. Wiz remains in the LTH with canoe. Its dry inside, so he can breathe.
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Lock dons ring, lock and barb swim up to the LTH. Barb goes inside and gets canoe out so lock can breathe. Refresh canoe's air.
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Lock and barb go further away, Lock gets in air pocket uses his tome to cast LTH, barb at his side.
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Barb gets ring and canoe, goes back to the previous LTH to bring mage over.
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Wizard uses canoe air pocket to cast LTH. Rinse repeat.
Should it work?
Best Answer
Ask Your DM. You have a number of questions here which don't have an official answer in the rulebook:
We could give you our best guesses at the answers (mostly, this seems unlikely to work very well), but these are the sorts of questions where your DM has to make a ruling, and anything we say would get overruled by your DM anyway.
(We can do some math regarding breathing: humans breathe "5-8 liters" of air per minute when not moving, or "12 liters" of air for light exertion. Multiply by two people, and assume you want to travel nine minutes undersea before spending 11 minutes casting the spell, so you need at least 400 liters of air. 400 liters of air displaces 900 pounds of water, so your canoe needs to weigh at least that much to balance it out.)
When your DM makes a ruling, they will consider a number of questions, including (1) whether this is practical, (2) whether it will advance the story, and (3) whether there's some other way you could search for this shipwreck that you should be doing instead.
There are easier ways to do this. There are lots of spells that would work better than Leomund's Tiny Hut for exploring underwater. In particular, please consider buying, learning, and casting Water Breathing which is the same level as Tiny Hut. Alter Self is also good.
When your DM asked you to search for a shipwreck, the method they probably had in mind was that you'd just take a boat out on the sea and go diving periodically. (Or perhaps you'd make deals with some merfolk for the location?)