Coral Reef

Warm, clear, shallow sea built up by corals into walls and gardens of colour. From a boat it shows as bands of turquoise, green and dark blue; underwater it crackles with the sound of shrimps and parrotfish crunching coral, and at low tide the exposed reef smells of salt and sun-baked weed. Fish of every shape and colour crowd it, with turtles, rays, octopuses and the sharks that patrol its edges. People fish it, dive it for pearls and shells, and live on the islands it shelters behind its walls. It is sharp enough to tear a hull and a danger to any ship that does not know the passes, and some of its creatures sting or poison. The Great Barrier Reef, off the coast of Australia, is the largest reef of this kind.

Hear it

Under the water: Underwater (In the album RPG Ambiences Vol. 2)
Diving: Diver (On Patreon, Guardian tier)
In the tropics: Tropical Coast (In the album RPG Ambiences Vol. 8)
Reefs and shoals: Booming Waves Cave (In the album RPG Ambiences Vol. 9)
Rain squall: Ship in Storm with Rain (In the album RPG Ambiences Vol. 1)
Becalmed: Calm Sea (In the album RPG Ambiences Vol. 8)
Waterspout: Stormy Sea (In the album RPG Ambiences Vol. 8)
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: scarce
Foraging: rich

Hazards

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.
Rain squall. A black wall of rain walks across the water and swallows the ship. For an hour the bow cannot be seen from the stern.
Becalmed. Not a breath of wind. The sails hang, the sea turns to oil, the sun beats on the deck and the water casks go down while the ship goes nowhere.
Waterspout. A grey funnel drops from a squall cloud to the sea and walks across the water, roaring. It can strip a ship’s rigging in a moment.
Water running low. The casks are low, or one has gone foul. Sea water only makes thirst worse; the crew is put on short water and watches the sky for rain.
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

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.
Rain squall. Heavy rain makes everything in it Lightly Obscured, gives Disadvantage on all Wisdom (Perception) checks and puts out open flames.
Becalmed. In a dead calm a ship cannot move under sail: it must be rowed, or it lies still. A ship that cannot be rowed makes no way that day.
Waterspout. (House rule) House rule: a ship that cannot get out of its path is hit. Everyone on deck makes a DC 15 Strength saving throw or takes 3d8 Bludgeoning damage and is thrown across the deck, or overboard on a d6 roll of 1 to 3. The ship takes 4d10 Bludgeoning damage and moves at half speed until its rigging is repaired.
Water running low. A Medium creature needs about 4 litres (1 gallon) of water a day. Drinking less than half of it brings 1 Exhaustion level at the end of the day, which does not go until a full day’s water is drunk.
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

A pearl diver knows a pass through the reef that no chart shows, and a pirate captain has taken her son to make her guide his ship through. She wants the ship to break on the coral, with her son off it first.
Some fishermen have started stunning fish with poison, and the reef is turning white. The elders want it stopped; the fishermen say the merchants who buy live fish leave them no choice, and name the merchants.
Once a year, on a calm night after the full moon, the reef spawns and the sea smells of fruit. An old woman wants to be rowed out to see it one last time, and she wants to be put in the water.
Divers hired to salvage a ship that broke on the reef keep bringing up lumps of coral grown around coins. Every lump is shaped like a closed hand, and the captain who hired them has stopped sleeping.

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)
Hunter Shark (a fight to fear, Challenge 2)

To gather

Snake Venom, Concentrated Venom

To fish

Coral Branch, Reef Octopus, Spiny Lobster, Pearl Oyster, Reef Shark, Sea Turtle

Nearby biomes

Coastal Shelf, Kelp Forest, Open Ocean, Deep Sea

Open it in Biomes — MGS Toolbox