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If a PC has a spell or item that allows them to breathe underwater, gives them gills, or whatever other means they might acquire to not drown, what is the maximum depth they could:

  • Descend to before buoyancy prevents you from going any deeper?

  • Descend before the pressure would become fatal?

Assuming we determine the maximum depth possible, how far could a character descend unassisted before their natural buoyancy prevents them from descending deeper?

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You can swim to any depth, but it might kill you

The Darkened Depths: Elemental Plane of Water

DMG, pg 56

The deeper extents of the plane, where no sunlight reaches, are called the Darkened Depths. Horrid creatures dwell here, and the absolute cold and crushing pressure mean a swift end to creatures accustomed to the surface or the Sea of Light. Krakens and other mighty leviathans claim this realm.

So it seems that it means a swift death for anyone who is accustomed to breathing above the surface or even in shallower waters, once you get to some depth below the water. This effect is explicitly nonmagical because the reasons for death are cited as "the absolute cold and crushing pressure."

There is no depth listed anywhere for where the Sea of Light gives way to the Darkened Depths, though. This is probably intentional, but if we use real world sunlight as our guide. Sunlight does not reach depths of below 1,000 meters (3,280 feet).

Also note that there are rules for dealing with Extreme Cold (if you consider this depth to be that cold, which it seems to be given the description):

DMG, pg 110, Extreme Cold

Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold climates

Strict RAW

DMG, pg 116-117

Swimming through deep water is similar to traveling at high altitudes, because of the water's pressure and cold temperature. For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.

So once your PC reaches a depth of 100 feet, every hour of travel counts as 2 hours for the purposes of determining exhaustion (note: you can travel for 8 hours per day without rolling for it), limiting you to 4 hours of actual travel time.

Once they reach a depth of 200 feet, swimming one hour in it counts as 4 hours, so you can only travel for 2 real hours before you have to start rolling.

Most PCs without a swim speed will typically have a speed of 30 ft, so they can swim 15 ft in 6 seconds. They can therefore reach a depth of 200 ft in 80 seconds, so this depth can be reached before you have to start making saves.

Additionally, assuming the same above (no swim speed, 30 ft walk speed), a PC can reach a depth of 3280 ft in less than 22 minutes, where they will presumably enter the Darkened Depths if they were in the Elemental Plane of Water. At this depth, they may very well die.

But the full extent of swimming at deep waters is... deeper

However, on top of the penalties to swimming in the deep, you have to also roll per hour of swimming. So if you were at a depth below 200 ft and swam for one hour, you are considered to have been swimming for 4 hours for the purposes of determining exhaustion. But you still have to roll a DC 10 Con save or gain one level of exhaustion:

DMG, pg 116

Unless aided by magic, a character can't swim for a full 8 hours per day. After each hour of swimming, a character must succeed on a DC 10 Constitution saving throw or gain one level of exhaustion.

This means, if you swim for two hours, you have to roll two DC 10 Constitution saves, and you are considered to have been traveling for 8 hours. If you swim for 3 hours under depths of 200 ft, then you must roll twice more:

  • One DC 10 Constitution saving throw for the extra hour of travel

  • One DC 14 Constitution saving throw for traveling the equivalent of 12 hours under the Forced March rules (DC = 10 + 1 per hour traveling above 8 hours).

And for every hour of swimming beyond that, you have to keep rolling twice. The first roll has the same DC throughout, while the second roll's DC increases by 4 each time.

The rules are the same for swimming under 100 ft, but the DCs eventually play out the same. You just take longer to get there.

Note: This is before taking into account the rules for Extreme Cold, if you were swimming in waters at or below 0 Farenheit.

None of these penalties are in effect for creatures with a swim speed

With the exception of the Darkened Depths, creatures with a swim speed do not suffer from these penalties.

DMG, pg 116

A creature that has a swimming speed -- including a character with a ring of swimming or similar magic -- can swim all day without penalty and uses the normal forced march rules in the Player's Handbook.

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In real life? Around 1,000 feet is the maximum done with scuba gear.

In game as far as the GM will allow it. There is AFAIK no damage for depth. So you could say the magic allowing them to breathe also gives them some protection from the water pressure.

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    \$\begingroup\$ Total Liquid Ventilation may be able to increase limits significantly, so if characters are magically breathing water, it is perfectly OK to say it also helps with depth. Hey, it works in real life, and also there is magic! \$\endgroup\$ – Mołot May 17 '17 at 21:16
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Storm King's Thunder contains pressure rules

In the 5e adventure module Storm King's Thunder, one of the final areas in the overarching adventure is

the storm giant kings' undersea stronghold Maelstrom.

"Maelstrom lies on the floor of the Trackless Sea, nearly 3,000 feet beneath the surface." (SKT, p. 202)

The best (read, safest) means for PCs to get there is by magic. However, if characters decide to try to go outside or if they take a boat (or other means of travel) to the area and

get sucked down to the depths by the great whirlpool that forms above Maelstrom,

they have to deal with the pressures.

Water Pressure

Creatures and vehicles at [spoiler] depth take 7 (2d6) bludgeoning damage per minute from water pressure unless they are adapted or built to withstand this environment. Storm giants, whales, sharks, crustaceans, and aquatic invertebrates are immune to water pressure at this depth, as are vehicles with a damage threshold of 10 or higher. Other creatures might be immune to the effect of water pressure, at your discretion.

A spell that allows one to breathe underwater provides no protection against the crushing effect of water pressure unless the spell's description says otherwise. Even an apparatus of Kwalish isn't designed to withstand water pressure at this depth. Creatures inside an apparatus of Kwalish are protected from the crushing effect of the pressure while the apparatus has at least 1 hit point. (SKT, p. 203)

Now, how you wish to adjudicate other depths is up to you. As someone else mentioned in another answer, the record for a SCUBA-only dive by a human is slightly over 1000'. Taking that into account along with the info presented in SKT, you may want to do 1d6 for every 1000' beyond the first.

   0-1000' - no damage
1001-2000' - 1d6/minute
2001-3000' - 2d6/minute
    ⋮

If you want smaller steps between damage increases, you might consider something like this:

1001-1500' - 1d6/min
1501-2000' - 1d8/min
2001-2500' - 1d10/min
2501-3000' - 2d6/min
3001-3500' - 2d8/min
    ⋮
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Not exactly sure, but more than 3,000 feet is certainly going to be a problem:

from https://www.pmel.noaa.gov/eoi/nemo1998/education/pressure.html

We often speak of pressure in terms of atmospheres. One atmosphere is equal to the weight of the earth's atmosphere at sea level, about 14.6 pounds per square inch. If you are at sea level, each square inch of your surface is subjected to a force of 14.6 pounds.

The pressure increases about one atmosphere for every 10 meters of water depth. At a depth of 5,000 meters the pressure will be approximately 500 atmospheres or 500 times greater than the pressure at sea level. That's a lot of pressure.

...

Dr. William Beebe was a pioneer in deep-sea exploration. With support from the National Geographic Society and the New York Zoological Society, Beebe constructed the Bathysphere (bathy = deep). In this steel sphere he would be lowered to depths of over 2,500 feet. The thick walled sphere was designed to withstand the great pressures of the ocean deep. The sphere had two thick quartz windows for viewing. To test the windows the bathysphere, unoccupied was lowered to 3,000 feet. When the great steel ball was hauled up, Beebe wrote.

"It was apparent that something was very wrong, and as the bathysphere swung clear I saw a needle of water shooting across the face of the port window. Weighing much more than she should have, she came over the side and was lowered to the deck. Looking through one of the good windows I could see that she was almost full of water. There were curious ripples on the top of the water, and I knew that the space above was filled with air, but such air as no human being could tolerate for a moment. Unceasingly the thin stream of water and air drove obliquely across the outer face of the quartz. I began to unscrew the giant wingbolt in the center of the door and after the first few turns, a strange high singing came forth, then a fine mist, steam -like in consistency, shot out, a needle of steam, then another and another. This warned me that I should have sensed when I looked through the window that the contents of the bathysphere were under terrific pressure. I cleared the deck in front of the door of everyone, staff and crew. One motion picture camera was placed on the upper deck and a second one close to, but well to one side of the bathysphere. Carefully, little by little, two of us turned the brass handles, soaked with the spray, and I listened as the high, musical tone of impatient confined elements gradually descended the scale, a quarter tone or less at each slight turn. Realizing what might happen; we leaned back as far as possible from the line of fire. Suddenly without the slightest warning, the bolt was torn from our hands and the mass of heavy metal shot across the deck like a shell from a gun. The trajectory was almost straight and the brass bolt hurtled into the steel winch thirty feet across the deck and sheared a half-inch notch gouged out by the harder metal. This was followed by a solid cylinder of water, which slackened after a while to a cataract, pouring out of the hole in the door, some air mingled with the water looking like hot steam. Instead of compressed air shooting through ice-cold water. If I had been in the way, I would have been decapitated. "

The pressures are great, indeed.

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    \$\begingroup\$ You're assuming D&D physics bear any resemblance to real-world physics. This isn't a good assumption ;) \$\endgroup\$ – Adeptus May 18 '17 at 6:40
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    \$\begingroup\$ But what else would they bear a resemblance to? It makes perfect sense to use real-world physics (and chemistry and biology) as much as is applicable, and make adjustments when magic requires it. \$\endgroup\$ – Phil Boncer May 18 '17 at 14:17
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    \$\begingroup\$ It really doesn't. DnD is a world where humanoids exhibit superhuman strength, the existence of bipedal giants as tall as buildings is not an anomaly, elves throw fireballs in the woods, and rogues completely avoiding said fireball when trapped with it in a 5'x5' room. Furthermore, a lot of these things are expressly 'not magic', but fall apart when we apply real world physics to them. \$\endgroup\$ – Pyrotechnical May 18 '17 at 14:37
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    \$\begingroup\$ I find it much easier to base some of my rules off of reality rather than having to spin new laws of physics every encounter. \$\endgroup\$ – Glen Pierce May 18 '17 at 15:28

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