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Basics

Core Rules

• If you don’t have a Swim speed, each 5 feet of movement takes an additional 5 more feet of movement.
• You can hold your breath for 1 + Con Mod (Min 0) Minutes

Automail

Automail usually doesn’t perform well in water. While it presents a pleasant tingling sensation when submerged in water, they are poorly designed for watery movement. Some Automail is actually designed for water, and will not impose the Automail swim speed penalty.

• Each piece of Automail requires an additional 5 feet of movement to move 5 feet.
  This effect stacks with the 5 foot extra if you don’t have a swim speed. You can be effectively dropped to zero movement, starting at 25 speed, it takes 10 ft of movement to move 5 feet, then factor in two automail legs and arms, and it takes 30 ft of movement to move 5 feet, making it impossible to swim without dashing.

Drowning

• When you run out of breath you drop unconscious. If Unconscious underwater you drop to 0 health, gain drowning status, and start dying.
• If you’re healed from unconscious or dying while underwater, at the start of your turn you’re out of breath and will drop back to unconscious, drowning, and dying without water breathing.
• If removed from water, the lungs are still filled with water, a Medical check DC 14 will remove the drowning status, or cast Destroy Water to magically clear their lungs. The Medical check clears the lungs enough to allow the person to breath, they must spend their next turn coughing up water, Destroy Water gets the person back on their feet immediately.
• Please note that Salt Water Aspiration causes some extra problems.

Salt Water Aspiration

The lungs are not well designed to handle water, and salt water on top of that adds another layer of problems. If a character starts drowning in salt water, they suffer from Salt Water Aspiration for an hour afterwards.
Duration: 1 hour

• difficulty breathing and chest pain. (Can’t hold breath for more than 3 rounds)
• a cough that produces phlegm (Can’t use Dive Suits or ADS)
• flu-like symptoms including body aches, exhaustion, fever, nausea and headache (Gain a level of Exhaustion at the start of this effect)
• pale skin
• shivering (Disadvantage on Dexterity checks)
  Salt Water Aspiration can be removed with a Lessor Restoration

Magic and Diving

Amulet of Adaptation:
Removes risk of Nitrogen Narcosis, grants advantage on Bends checks.
Alter Self: Water Adaptation:
Removes risk of Nitrogen Narcosis, grants advantage on Bends checks.
Water Breathing:
Removes risk of Nitrogen Narcosis, but not the Bends as that’s pressure related. Leaving the primary concern crush depth and temperature damage.
Freedom of Movement:
Removes the extra five feet of extra movement required without a swim speed. Also removes the negatives to movement from Automail.
Create / Destroy Water:
Can be cast to remove water from a persons lungs, removing the Drowning status. However it doesn’t replace the water with breathable oxygen. If a person is drowning, you can cast destroy water and provide them with an oxygen source they’ll be able to continue acting on their next turn. Destroyed Salt Water still causes Salt Water Aspiration.
Control Water:
You can remove water from a character’s lungs, if it’s salt water they don’t suffer from Salt Water Aspiration.
Water Walk
Character surfaces at 60 ft per turn, this is extremely likely to cause The Bends.

Wet Suits, Dive Suits, and Atmospheric Dive Suits

Wet Suits are special armor, due to their tight design they can fit under any other form of armor.

Armor and Swimming

No Armor:
Without armor and if proficient in Athletic, unless the waters are turbulent or dangerous you can move without making an Athletics Check.

Light Armor:
Swimming with Light Armor requires an Athletics Check DC 5 to be able to move. Most trained adventurers can swim in light armor no problem.

Medium Armor:
Swimming with Medium Armor requires an Athletics Check DC 10 to be able to move.

Heavy Armor:
Swimming with Heavy Armor requires an Athletics Check DC 15 to be able to move.

Dangers

Temperature

Water conducts heat far better than air, thus you lose body heat far faster if you’re unprotected. Wearing a Wet Suit provides advantage to Cold Aquatic Environment Checks, while wearing the heavier Dive Suit or an ADS will provide a flat + 5 bonus to these cold Aquatic Environment checks.
A layer of grease or fat smeared over a character can also provide a + 5 bonus to cold Aquatic Environment checks due to it’s water repellent nature. This can be stacked with Dive Suit or ADS bonus, although requires an hour of cleaning afterwards.
Checks are made every minute

• Tropical or Warm water: No check
• Cold: Constitution DC 10 | 1d6 cold damage per minute
• Frigid: Constitution DC 15 | 2d6 cold damage per minute
• Icy: Constitution DC 20 | 4d6 cold damage per minute

Ice

Water freezes below Icy temperatures, however since ice floats to the surface, it provides an insulating barrier. Most bodies of water don’t freeze through, the top will be ice but the water underneath will be near freezing (Icy) year round.
Breaking Ice (From Above): A Strength check is required to break surface ice, if the player is armed with a piercing weapon, they gain advantage on the strength check.
Breaking Ice (From Below): Due to the resistance water provides, breaking ice from below is difficult, players make a Strength check at Disadvantage is required to break the ice. If the player is armed with a piercing weapon, they can ignore Disadvantage.
Thickness: Check (Top Weight Capacity)

• Film (1"): Strength DC 4. (Will break)
• Sheet (2"): Strength DC 8. (One Light person with distributed weight, skis or snow shoes)
• Thin (3"): Strength DC 12. (One person on foot)
• Medium (4"): Strength DC 16. (Group of people in single file)
• Thick (6"): Strength DC 22. (Breaks above 1 Ton)
• Dense (8"): Strength DC 28. (Breaks above 2 Tons)
• Solid (10"): Strength DC 34. (Breaks above 3 Tons)

Pressure & Crush Depth

Pressure is all about depth acclimation in terms of danger to a character. When a character enters a new depth range, they must make a Constitution Save, success results in them acclimating to that depth and they no longer need to make saves; while failure results in them taking the depth effect and making the save again next turn. Depth effect is determined by looking at you’re save roll, on an Even it’s damage, on an Odd it’s a level of Nitrogen Narcosis.

• 0 – 100 feet: No check | no effects.
• 101 – 150 feet: Constitution DC 4 | 1d6 damage
• 151 – 200 feet: Constitution DC 8 | 2d6 damage
• 201 – 300 feet: Constitution DC 12 | 3d6 damage
• 301 – 400 feet: Constitution DC 16 | 4d6 damage
• 401 – 600 feet: Constitution DC 20 | 5d6 damage
• 601 – 800 feet: Constitution DC 24 | 6d6 damage
• 801 – 1000 feet: Constitution DC 28 | 7d6 damage
• 1000 + feet: Constitution DC 32 | 8d6 damage

The Bends

Ascending to quickly causes the oxygen bubbles in the blood to expand as pressure decreases. Ascending to quickly and you’ll be looking at:

• Gaining a level of Exhaustion.
• Loss of consciousness.
• Seizures.
• Paralysis.
• Death.

Nitrogen Narcosis

Also known as Rapture of the Deep. Has the effects of laughing gas, and can start as high as 30 meters (99 feet) from the surface. It can quickly be relieved by simply reducing depth. It’s akin to Altitude sickness, and means to prevent it have not been found. (In the real world, it’s combated by mixing Helium into the dive mix.) A Water breathing spell will also allow you to ignore this problem, as Nitrogen Narcosis is caused by O2 – Nitrogen imbalance. Reversing the latest change in depth will remove a level of Nitrogen Narcosis.
Surfacing above 60 feet deep, will clear all Nitrogen Narcosis levels.

Nitrogen Narcosis Levels

1. Gain disadvantage on all skills your not proficient in, disadvantage on Intelligence checks and skills, and Mild Euphoria.
2. Gain disadvantage on Wisdom checks and skills, occasional Loquacity or Anxiety.
3. Disadvantage on Initiative, Sleepiness, Hallucinations, occasional dizziness, occasional Hysteria or Terror.
4. Disadvantage on Dexterity, Loss of memory, increased excitability.
5. Increased intensity of vision and hearing, Disorganization of the sense of time, sense of impending blackout, euphoria, dizziness, levitation, manic, or depressive states.
6. Unconsciousness
7. Death