Dungeons & Dragons adventures can take place almost anywhere, but adventures at sea are an evergreen favorite. Players and Dungeon Masters alike are almost always keen to see ships set sail for distant ports, and these kinds of adventures often feature plenty of time in coastal areas.
There are plenty of monsters that are comfortable on land, and many are purely aquatic, but there are certain creatures that are perfectly at home on the border between land and sea. These kinds of adventures are a great opportunity to see a diverse array of D&D monsters, helping to add to their popularity.
Updated by Lauren Turner on April 14, 2023: Many D&D players enjoy campaigns set near the coast. As such, we've revamped this list with more information to make coastal adventuring more rewarding.
15 Sahuagin
CR: 1/2
Sahuagin are fish-like creatures that are referred to as sea devils by some coastal residents due to their tendency to prey on seaside settlements. They have a blood frenzy ability that grants them advantage on melee attacks against enemies that have taken any amount of damage, which makes them a powerful foe.
Some sahuagin take up spellcasting as dark clerics, while others grow to immense sizes and rule as brutal barons. With their antagonistic relationship with shipping and coastal towns, sahuagin are obvious enemies for a party on the coast — but with more complexity and variety than a great many monsters in Fifth Edition.
14 Merfolk & Merrow
CR: 1/8, CR: 2
Merfolk are common in many mythologies throughout the world, as they represent a familiar yet alien entity. Merfolk can be good or evil just like any other sapient creature, meaning they could easily be enemies or allies in a D&D adventure set by the sea.
Merrow, by contrast, are something far less human. They are merfolk transformed by the corrupting nature of the demon lord Demogorgon. They're malevolent and cruel creatures that can serve on their own as a threat or give the player characters a reason to side with merfolk against a common foe.
13 Giant Shark
CR: 5
When compared to the sapient D&D creatures and monsters who inhabit the shallows, a bigger version of something found on Earth can seem underwhelming. However, the giant shark statblock for Fifth Edition is a remarkably powerful and intimidating one, well-suited for a terrifying coastal monstrosity.
Whether a side encounter during another adventure or the focus of a Jaws-esque quest to make the coast safer, giant sharks can make surprisingly dangerous opponents for player characters. For a less fantastical coastal adventure, they are a solid choice.
12 Sea Hag
CR: 2
Hags are powerful fey spellcasters, but sea hags are the weakest among them. Covered in slimy scales and draped in fetid seaweed, these creatures often conceal their true nature with an illusory veil. They could approach the adventurers as a friendly or hostile face to kickstart a D&D adventure.
Sea hags lack the spellcasting abilities of their more powerful relatives, though they do boast the powerful Death Glare ability, which can reduce a target to zero hit points immediately. Sea hags, like all hags, can join together in a coven to gain access to much more powerful abilities, allowing for a suitable end-of-adventure battle from seemingly weak creatures.
11 Fathomer
CR: 2
Cults and cultists are ready-made villains for almost any adventure in D&D, and Ghosts of Saltmarsh ensures that there are some perfectly suited for coastal adventures as well. Fathomers are spellcasting humans who can turn into enormous watery snakes thanks to pacts with ancient evil, the ideal threat to catch players off-guard.
A major boon of the fathomer is their versatility. They can easily menace the player characters in the shallows, but they can also track, attack, or plot against them on land. They're less restricted than other coastal creatures while still fitting the environment perfectly.
10 Tortle
CR: 1/4
Although not the best-suited aquatic creatures, lacking an innate swimming speed, tortles can handle themselves well by the coast. They have the ability to hold their breath for an hour, allowing them to function naturally in areas where other humanoid creatures would need to augment themselves with magic.
As such, a tortle or even a band of them could well live on the coast, exploring the murky depths beyond. Although they're stated to lean toward lawful good as a race, there is no reason player characters couldn't come across an evil or criminal group of them — or even simply find themselves opposed by good or noble tortles.
9 Koalinth
CR: 1/2
Many coastal creatures tend to be of the wild or cultish variety without much emphasis on order or tactics. Koalinths are very different, being aquatic versions of the classic D&D hobgoblin. Although they are noted for their aggressive nature, they retain the lawful leanings of their surface kin, as well several other traits.
If a DM gets creative with the hobgoblin variants presented throughout various books in 5e, koalinths can be used to create a unique martial threat to the players on the coast. In particular, they can be easier to reason or negotiate with than most monsters, being a sapient and rational species.
8 Sea Spawn
CR: 1
Legends say that the sea spawn are those lost sailors who never returned from their voyages. These creatures find themselves drawn to powerful monstrosities within the sea, acting as their footsoldiers and servants should they be needed.
Sea spawn grow to resemble various sea creatures as they warp and change and have access to powerful bite attacks, poisonous quills, and long-reaching tentacles. Versatile and often in league with greater powers, they make good threats for low-level coastal campaigns or good henchmen for higher levels.
7 Wastrilith
CR: 13
D&D's powerful demons can be found in almost any ecological niche, and wastriliths dominate the aquatic realms. They are capable of fouling the water around them, whether it be in the ocean or in containers. This foul water poisons anyone who swims in it or consumes it, while demons heal when they drink it.
In direct combat, wastriliths can fight with their vicious claws, create difficult terrain via rippling undertows, and even launch spouts of caustic water to destroy their enemies. Adventurers fighting them in coastal waters are at a terrain disadvantage, creating a unique dynamic for players who want to hunt them down.
6 Kraken Priest
CR: 5
Coastal adventures can often serve as one component of — or a prelude to — a story arc or entire campaign taking place within the deeper sea. Many of the ocean's greatest threats are poorly suited to the shallows, but that isn't to say they can't have heralds or servants who go there to give the players an inkling of what they're dealing with.
Kraken priests are creatures who've turned to worshipping krakens out of fear and awe and have been given powers as a result. One or more could easily travel to the coast to further a kraken's goals. This can introduce the PCs to a kraken villain without putting them in the firing line of a monstrosity capable of challenging max-level characters.
5 Deep Scion
CR: 3
Deep Scions are shapeshifters that are created when desperate, drowning soldiers make deals with foul aquatic entities like krakens or marids in their dying moments. They return as foul, tentacled humanoids that work in secret to advance their master's agendas.
Deep scions are powerful, posing a threat on their own to even D&D characters up to about the fifth level. As well as making good monsters or henchmen, they can also be particularly effective enemies for warlocks — particularly Fathomless warlocks — serving as a dark reflection of a fairly common player character archetype.
4 Pirate
CR: 1/8
Setting a D&D adventure near the coast probably means the party will spend a good amount of time on ships, and any good ship-based campaign should feature some good old-fashioned swashbuckling. NPC pirates can have a wide variety of skills, from nimble swashbucklers and scouts to powerful warlocks and wizards.
Pirate crews can be a menace to ships or settlements, a force well-equipped to strike rapidly, overwhelm their foes, and then leave. Whether they're defending an area from them or boarding the pirates' ship themselves to defeat them once and for all, including pirates in a coastal adventure is an excellent way to let players live out their buccaneer fantasies.
3 Storm Giant
CR: 13
D&D giants come in several different types, but the strongest among them is the storm giants. These enormous beings live deep underwater or high in the sky, meaning they are difficult for any mortals to come in contact with. They are gifted prophets and have an innate magical ability, making them powerful allies and dangerous foes.
Most storm giants are chaotic good in alignment, but this doesn't mean they'll always be on the side of the party. They are capricious and quick to anger, and as such, they can serve as fine antagonists despite their typical morality.
2 Aboleth
CR: 10
Aboleths are one of the most intimidating and frightening aquatic creatures in all of D&D, ancient aberrations with immense psionic powers. They're most commonly sighted in the waters of the Underdark, far away from all living beings. But an aboleth settling on the coast near a humanoid settlement is a reason for fear.
Aboleths secrete a substance that transforms those caught in it into helpless puppets. If an aboleth is by a coastal town, this puts pressure on the player characters to deal with it before it can take over the entire town — while also leaving them entirely unsure of who they can trust and who might be an enemy.
1 Bronze Dragon
CR: 2-28
As metallic dragons, bronze dragons are much more likely to be good-aligned than their chromatic counterparts. They love to make their homes in coastal cliffs, watching the passing ships with great interest. While bronze dragons generally stand up for the downtrodden and oppose tyranny, it's easy to imagine one being manipulated, tricked, or enchanted into attacking people it would otherwise protect.
Dragons are some of the biggest and most iconic threats in D&D, and most players relish the chance to challenge one in any terrain. In the case of a misguided bronze dragon, it provides a unique challenge for a party to bring one down without killing it, sure to create a memorable adventure.