If you're coming into this series here, this is a set of rules for underwater play. Everything from a scuba-diving one-shot to an Atlantean adventure to a whole campaign taking place underneath the waves. I'm assuming medieval/early modern diving technology being common, plus some cutting edge stuff the PCs have a chance to get their hands on at greater prices. Fighting against the logistical difficulties of the deep ocean s the big challenge here, so I go forward assuming no underwater races, and no easy access to water-breathing.
Happy diving.
Rules
Melee
Bludgeoning weapons (hammers, maces) take -6 to damage.
Slashing weapons (axes, swords)take -3.
Piercing weapons (daggers, spears) take no penalty.
Ranged
Throwing daggers, and other such weapons deal 1 damage to targets 5’ away.
Javelins, bows and crossbows take -1 damage for each 5’ beyond the first 10’.
Movement
An athletic human, using assisted diving technology such as fins, can move 30’ in a standard round while carrying standard equipment. Moving with substantially excess weight lowers movement to 1/2x standard.
Characters with no armor add their DEX modifier to their armor class.
Light Armor: As Leather. These include padded diving suits and armors made from sea-animal skins (seal, shark, etc).
Medium Armor: As Chain. Includes harnesses carrying thin metal plates and light ring shirts.
Heavy Armor: As Plate. Includes heavy diving suits, designed for deep submergence.
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| Laurent Ballesta, Sodwana Bay |
Some breathing mechanisms include:
Unassisted: Holding one’s breath lasts for a minute of activity, allowing for a dive down to 150’ and an immediate return to the surface. Narcosis sets in at 100’.
Diving Bell: Bell-shaped chamber pumping air from the surface with a hose. Provides a source of air without returning to the surface. Costs 30gp.
Diving Cylinder: Cylindrical metal containers that contain compressed gases. Costs about 10gp and takes up one inventory slot, empty or full.
Compressed Air: Can be compressed for free with a pump on the surface. Lasts for 1 hour. Narcosis still sets in at 100’.
Diver’s Gas: Mixes of gases with funny names produced by alchemists and wizards with funnier names. More expensive, costing some 1gp per tank, but allows for diving down to 300’.
Potions: Magical concoctions, whether in the form of a draught, pill, salve, etc. which have a particular beneficial effect, such as water breathing, or bends resistance, or better insulation.
Magic Items: Some magic items will offer resistance to narcosis or depressurization up to certain depths, specified along with each item.
Mutations: Biomagical adaptations to the undersea environment, enhancing a creature’s abilities; anything from improving lung capacity to creating gills, allowing for indefinite time underwater.
Eating and Drinking Underwater
It is possible to eat and drink underwater with some practice. Waterskins are replaced with flasks, and rations are constructed to be eaten underwater with minimal difficulty.
Resting Underwater
It is possible to take a 1 hour short rest (a lunch), underwater, with all the benefits that brings, so long as the party’s air supply is sufficient.
A long rest (good night’s rest) is not possible while submerged. The players must either return to the surface or else find some dry location, like a cave or underwater habitat with air and a heat source.
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| u/Himerly |
Combat
Melee Weapons: Most melee weapons are unsuited to underwater combat due to increased drag. Piercing weapons are the least affected. Blunt weapons are slowed to the point of near uselessness. This is a rule, implemented in various forms, by most fantasy RPGs with underwater rules. For one, it makes real-life sense. For another, it gives the players a mechanical reason to replicate the trident and harpoon-gun aesthetic we're working to create here.
Ranged Weapons: Almost all normal ranged weapons are ineffective underwater. Throwing weapons, especially small ones, are nearly useless. Further, strung weapons, such as crossbows, must be specially treated, or else become inoperable in 1d4 hours of submergence.
Specialized Weapons: Certain weapons are favored by underwater combatants. Spears and daggers are the least cumbersome melee weapons. Nets are very useful in constraining the movement of humanoid combatants. Ancient weapons using water pressure and even firepowder can be recovered, which fling projectiles at great speed.
I ran the numbers, and the 30' movement figure actually isn't unrealistic assuming a 6 second round. In a 10 second round, it's just right. A human with fins can reach up to 1.5m/s, corresponding to just less than 30' in a round. Michael Phelps, at his top speed of about 5.5 miles per hour, approaching 50' in a 6 second round. I'm not assuming the players are Olympians, but they should be competent and well equipped. As a result, I'm comfortable using the standard figure.
Armor
Most armors are unsuited to being worn underwater. Plate will make underwater movement impossible, and even chain will weigh you down. With rust and wear, different kinds of armor are needed. Armors made and worn underwater tend to be much lighter than those on the surface, and are focused primarily on preventing piercing damage.
My visual image for a lot of this comes from Thunderball and sponge-diving, corresponding to no and light armor respectively. Medium armor starts to get heavy-duty, but still built for an unerwater environment where mobility and piercing damage is king. Heavy armor is tougher to move in, but provides a ton of protection, both from attacks and the environment. I'm imagining Mothership's vaccsuits.
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| Point Nemo, the farthest point from land in the world Almost no ocean life survives here |
Rest and Survival
Breathing Underwater
When operating underwater, the most limited resource is breathable air. The party may find some pockets in underwater caves or artificial structures, but they can’t rely on those. The air they bring with them is their most essential resource, and running out deep beneath the waves is a sure death.
The PCs are assumed to be competent, but not supernaturally talented. They can maintain one minute of activity with their last breath, enough to swim 300’. An immediate ascent from <300’ to the surface may well save their lives, but will likely result in the bends.
This module does not assume the technology or magic level of the campaign world. Some methods breathing mechanisms, such as scuba gear, bathyspheres and diving helms, may be available technology or scavenged from an ancient civilization. Others, like potions, mutations and spells, vary depending on your campaign’s approach to magic.
Breathing mechanisms have three characteristics: rarity, duration and depth. Some methods of water breathing are more commonly available, while others are rare and expensive. Some last a long time, others very little. Some allow for exploration at great depths before suffering narcosis, others will only permit shallow exploration. At no point should a mechanism that is cheap, long-lasting and effective be present; it would be like having unlimited, bright light or good food underground.
Potions: Consumable magic items are a must, and an underwater environment makes you reconsider the form they come in. Glass vials filled with liquid wouldn't be too convenient. Other forms, like a breath of magic gas, performance enhancing pills, healing serum injected directly into the body.
Magic Items: The possibilities for magic items are great. I'm thinking shark-skin cloaks that allow transformation, enchanted diving helms, consumable figurines that break to absorb the bends.
Mutations: While these may seem very powerful, they are permissible with caveats. For one, they are likely rare and risky; the whole party won’t be able to try them, and those who do are taking a serious risk. Further, they carry penalties outside of water. The gilled PC now cannot breathe on the surface without a special mechanism!
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