Running Underwater Adventures

Above its domes the gulfs accumulate.

Far up, the sea-gales blare their bitter screed:
But here the buried waters take no heed—
Deaf, and with welded lips pressed down by weight
Of the upper ocean. Dim, interminate,
In cities over-webbed with somber weed,
Where galleons crumble and the krakens breed,
The slow tide coils through sunken court and gate.

- Clark Ashton Smith, “Atlantis”

Several months back I had, thanks to the actions of a player in picking up a very-obviously cursed goddess statue, the chance to finally run a full-on underwater adventure. In my many years of running D&D this was the first time I have designed and run an underwater adventure and I found it challenging and interesting in equal measure.

Having thrown the idol into a specific spot under the urging of fever dreams (and yes the heavy hand of geas), the party returned 40 days later as commanded to the site. A massive black pylon had--naturally it's D&D--risen from the sea. A long-drowned city with wide avenues and step pyramids leading up to a large glowing central dome could be vaguely made out in the depths below. Sweet water seeping from the naughty bits of  a now gigantic (and lewd enough to make me still feel awkward) idol inside the pylon allowed them to breathe sea water and away they went.

The party made it through a couple sessions and then some of their own character-driven goals called them away (such is sandbox life). This morning I randomly refound my notes for running the adventure and thought they might be of interest to a broader audience. So with some clean-up and context here they are. 

Sources for D&D Underwater Adventures

Blackmoor OD&D supplement, pages 48-49. Some rules and guidelines for running aquatic adventures. Best line: “To entice the players into the water is the easy part: simply leave maps, create legends, or sprinkle in clues in the form of bizarre artifacts. Adventurers are notoriously gullible and easily duped or led astray.”

Dungeon Masters Guide 1^st edition, pages 55-57. Nice comprehensive set of guidelines, I used this as my starting point.

“Watery Words to the Wise”, Dragon magazine #48. Interesting overview of what an aquatic civilization of Mer-people would be like, but more useful for its thoughts on using a hex map for 3D tactical affairs.

Dealing with the 3D Environment

One of the hardest things to remember and plan for is the 3D environment. I found that it was helpful to keep two sheets of graph paper handy to help keep track of the relative positions of things:

1. A blank sheet that shows the relative “marching” order of positions vertically. This especially became necessary as the party had some encumbered players lumbering over the ocean floor (see sinking rules below) and others “flying” around at varying depths over and around these characters. This also especially handy when encounters are met in open water. Using clock descriptions is helpful (“the sharks are coming in at “two o'clock high”) when describing those positions.

2. A rough cross section of whatever static sites you have. Sites can be approached from a wider set of directions and it's fun (and challenging) to play this freedom up with multiple vertical entry points and elements. I had a little step-pyramid with a top entry, chimney and a bottom floor entry. Having an overall sense of where everything is vertically is very important to staying oriented.

Movement

Characters encumbered with more than 20 pounds of gear and/or metal-armored characters sink to the bottom. Characters with high STR or CON (13 and over) are able to swim up 10 feet for two rounds every other turn (and will naturally sink again if there is nothing solid below their feet). Others will need to shed their weight before being able to swim or float upwards. (Bundles of gear can be pulled upwards by ropes if the character is out of the water.)

Unencumbered characters swim at their indoor, dungeon movement rates horizontally or vertically. Encumbered (as above) characters move at 60 feet per round.

Fly, Levitate, Free Action, Spider Climb (when on a surface) spells and abilities will allow the user to move at their normal land speeds without sinking for the length of the effect.

Water Pressure

Having blown out an ear drum on an overly-quick descent on a dive in Cozumel I am painfully aware of the effects of pressure on the human anatomy. At depths lower than 100 feet (30 meters or so) nitrogen narcosis becomes a real thing for humans without protective gear.

Because this is a fantasy game (and it it's fun to be able to run adventures with a wider tolerance of depths) I assume that Water Breathing and similar magics at least double the depth that pressure can be withstood. Compression/decompression issues are totally handwaved.

Characters dropping below the depth can withstand the pressure for a round. After that they will suffer the same effects as being seriously intoxicated for 1d4 rounds and then start to take 1d4 hp damage each round after.

Visibility

Light, refraction, physiology, etc conspire against us seeing far underwater unaided. While I didn't want to totally handwave this, the rules given in AD&D are way too restrictive (100 feet up to a depth of the same). A DM especially with a large area like my underwater ruined city is hard pressed to be able to signal interesting sites and thus reduces meaningful exploration choices. My handwave was similar to that of the depth considerations and I made a simple chart to give a wider range:

Depth	Effective Visibility
0-100 feet	200 feet
101-150 feet	100 feet
150-200 feet	50 feet

Naturally lighting will reduce this especially inside structures, caves, crevasses, kelp forests etc. In that case I limited visibility to the normal effects of artificial lighting (light spells mostly) or

infravision.

Combat

Aquatic critters auto-win initiative against the land-born.

Most missile, bashing, and slashing weapons are useless in this environment. Piercing weapons such as spears, tridents, daggers are highly effective however and will deal out normal damage. Reduce swords to 1d4 damage to represent them being limited to stabbing.

Spells

The vocal component of spell-casting is assumed to come with the ability to breathe water. Page 57 of the DMG has a relatively complete run down of restrictions and changes to spells. Significant changes in my own were to increase the damage of electricity-based (150%) spells.