Kelp Forest

Cold coastal waters where giant seaweed rises from the rocky bottom to the surface like a submerged wood. Seen from above, it is a brown mat rocking on the swell; below, the light comes down green and golden through the swaying stalks, and on the beach the cast-up weed smells of iodine and rot. Fish shelter among the stalks, crabs and urchins graze the floor, and seals and sea otters hunt through it. People cut the kelp for soap, glass and fertiliser, fish its edges and set lobster pots among its roots. Divers and swimmers can get tangled in it, and a boat’s rudder can be caught fast at the worst moment. The kelp forests off the coast of California, home to sea otters, are forests of this kind.

Hear it

Under the water: Underwater (In the album RPG Ambiences Vol. 2)
Diving: Diver (On Patreon, Guardian tier)
Calm: Calm Sea (In the album RPG Ambiences Vol. 8)
Sea fog: Sailing Ship (In the album RPG Ambiences Vol. 1)
Reefs and shoals: Booming Waves Cave (In the album RPG Ambiences Vol. 9)
Storm at sea: Ship in Storm without Rain (In the album RPG Ambiences Vol. 1)
Frigid water: Underwater and Ice (In the album RPG Ambiences Vol. 7)
Man overboard: Rough Sea (In the album RPG Ambiences Vol. 8)

For the table

Ground: At sea: Sailing ship: 77 km a day, Warship: 96 km a day, Galley: 154 km a day, Longship: 115 km a day
Water: by season
Foraging: rich

Hazards

Fouled in the weed. The kelp canopy is thick as a hedge. It wraps rudders and oars, holds a boat like a hand, and hides the rocks it grows on.
Sea fog. A white fog lies on the water. Bells and shouts carry strangely, another ship can loom up a cable away, and the coast is wherever the lead line says it is.
Reefs and shoals. Rock or coral a fathom under the surface, a sandbank that moved since the chart was drawn. A ship that strikes may float off at the next tide, or open her hull.
Storm at sea. The wind rises for hours, the sea builds into grey hills and the crew reefs the sails or runs before it. No one sleeps dry; the ship goes where the storm sends it, not where the chart says.
Frigid water. In cold seas a fall overboard is a race against the cold. The swimmer loses the use of hands, then of legs, long before drowning.
Going under. A wave breaks over a swimmer, a diver stays down too long, a sinking hull drags someone with it. Air runs out faster than anyone thinks.
Man overboard. A green sea sweeps the deck, or a foot slips at the rail, and someone is in the water. By the time the ship comes about, the swimmer is a dot among the waves.

D&D 5e

Plays as: waterborne, by the ship pace at most
Checks: forage: Survival DC 15 · find the way: DC 10 · search: DC 15
Creatures are spotted at: 6d6 × 10 ft

Fouled in the weed. (House rule) House rule: a ship or boat that pushes into the canopy moves at half speed, and its rudder or oars are fouled on a d6 roll of 1 each hour: a DC 12 Strength (Athletics) check over the side, with a blade, clears it in 10 minutes.
Sea fog. Heavy fog is Heavily Obscured: a creature trying to see something in it has the Blinded condition. Patchy fog is Lightly Obscured: Disadvantage on Wisdom (Perception) checks that rely on sight.
Reefs and shoals. (House rule) House rule: the lookout makes a Wisdom (Perception) check, DC 15 by day, 20 at night or in rain. On a failure the ship strikes: it takes 4d10 Bludgeoning damage (reduced by its damage threshold as usual) and stops until the crew warps it off, about an hour of work per 10 damage taken.
Storm at sea. Strong wind gives Disadvantage on ranged weapon attacks, puts out open flames and scatters fog; a flyer must land at the end of its turn or fall. A ship sailing into a strong wind moves at half speed.
Frigid water. A creature can stay in frigid water for as many minutes as its Constitution score. Each minute after that, it makes a DC 10 Constitution save or gains 1 Exhaustion level. Cold resistance or immunity, or being adapted to icy water, passes automatically.
Going under. A creature holds its breath for 1 + its Constitution modifier minutes (at least 30 seconds). After that it gains 1 Exhaustion level at the end of each of its turns until it can breathe; it then loses all the Exhaustion it gained this way.
Man overboard. Swimming costs 1 extra foot of movement for each foot moved (none with a Swim Speed). In rough water the GM may ask a DC 15 Strength (Athletics) check to move at all.

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

Adventure hooks

Kelp cutters working for a soap boiler have found a fishing boat sunk among the stalks, its crew gone and its hatch bolted from the inside. The boiler wants the kelp cut anyway; the cutters want someone else to open the hatch.
Sea otters have been stealing the village’s clam rakes and hiding them in the kelp. The village offered a reward, and the rakes now come back one at a time, each time a fish is left on the jetty.
A smuggler sinks his kegs on weighted lines deep in the kelp and fishes them out once the customs cutter has passed. His diver drowned last week, and the next delivery is tomorrow night.
A storm has torn up half the kelp forest and the beach is piled high with it. Sea urchins are eating what is left, and the fishermen whose fish shelter there want help clearing them before nothing remains.

Animals

Sea Otter (harmless, in colonies, by day)

Creatures

Merfolk (a threat to the unwary, Challenge 1/8)
Giant Seahorse (a threat to the unwary, Challenge 1/2)
Sahuagin (a threat to the unwary, Challenge 1/2)
Giant Octopus (a fight to fear, Challenge 1)
Merrow (a fight to fear, Challenge 2)
Sea Hag (a fight to fear, Challenge 2)

To gather

Sea Otter Pelt, Iodine Powder

To fish

Kelp Bundle, Reef Octopus

Nearby biomes

Coastal Shelf, Coral Reef, Open Ocean, Deep Sea

Open it in Biomes — MGS Toolbox