The Best Monsters For An Aquatic Setting In DnD

Aquatic adventures, whether they be on a ship, underwater, or even in the elemental plane of water, can be one of the most memorable adventures in Dungeons & Dragons. Many modules are set around exploring uncharted waters, and they are one of the favorite biomes for players to experience.

Setting up encounters for aquatic settings might require a lot of preparation and research. But knowing some of the most common and interesting inhabitants of these biomes can help you come up with various encounter ideas for your game.

Updated on May 17, 2026, by Jerel Levy: We've added even more creatures for an aquatic setting. This time, we've gone a bit prehistoric thanks to Dr. Dhrolin's Dictionary of Dinosaurs. Whether you're going full prehistoric or not, these aquatic creatures are sure to shake up your campaign even further. They create some unique underwater encounters your players won't soon forget. Here are even more sea monsters to use in your 5E campaign.

27 Wretchglow, The Last Light

The Scariest Underwater Encounter

This deadly sea creature brings both a prehistoric and magical nature to your aquatic adventure. If you're looking for a true sea monster, Wretchglow is the perfect option. Reminiscent of the scariest deep-sea monsters your players can conjure up, they'll be shocked as they encounter the pulsing bioluminescent light that leads to the jaws of death.

With legendary actions to direct the currents, preventing the party from making a quick escape, as well as attacking with its hooked claw and needle-toothed jaws, it's a Huge beast they'll never forget, if they live through the encounter. Even better, it can be hidden, so it's a great boss battle to sneak against the party as they traverse the deep depths.

26 Mosasaurus

An Intimidating Presence

The Mosasaurus is another large creature that any party would freeze if they encountered it underwater. With a Resistance to Cold, Lightning, and Thunder damage, spellcasters might have a hard time dealing with this Huge creature. Its Stormrider ability also gives it Immunity to Control Water and Tsunami effects, so don't think they'll be able to just maneuver it out of their way either.

The Mosasaurus makes both a Piercing attack and an attack that deals Thunder damage, so it'll be sure to ignore someone's Resistance in the party. It also has Tough Hide and a decent AC, so good luck trying to deal with this dangerous creature.

25 Deinosuchus

Resistant To Everything

Gapple and restrain the party with a battle similar to fighting a gator, thanks ot the Deinosuchus. It's a hybrid creature, so you can use it in aquatic settings that are underwater and shallow or closer to land. It can hold its breath for an hour, and when underwater, it gets Advantage on Stealth checks.

The most dangerous part of the Deinosuchus is its Hyper Resilience. It's Immune to Poison and resistant to all damage. With a hardy 18 AC and a decent health pool, it'll definitely be a DPS or strategy check for your aquatic campaign.

24 Seth The Shapeshifting Dragon

Don't Be Fooled By His Origin

This powerful dragon is separate from the others on this list as he's from the release of Misplaced Monsters: Volume One. In partnership with Extra Life for Children’s Miracle Network Hospitals to raise funds, several kids got to design their own monsters, which were eventually turned into full-stat monster options for any campaign.

Seth the Shapeshifting Dragon is a very powerful dragon that is completely invisible while submerged in water, has a breath weapon that shoots frigid water, and has ridiculous bursts of speed to catch any creature trying to get away. Seth can change shape at will, has super speed that gives disadvantage to any creature trying to attack him while it's active, and his Riptide Breath shoots in a cone that'll deal heavy damage to anyone caught in it. He's a monster of a beast that'll present a true challenge.

23 Lorwyn Merrow

The Sly And Cunning Cousins

Unlike their vicious cousins, the Merrow of Lorywn, found in Lorwyn: First Light, are described as prideful and diplomatic. They're great at language and can take anyone on in a fight of words, making them great for a campaign where you need to make a connection between sea dwellers and land dwellers. They're also great for presenting a fun mix up to how aquatic creatures are shown and treated.

The Shadowmoor and Doublespeaker Merrow are cunning and will easily trick an adventuring party out of everything they own if given the chance. You'll lose your treasure and your life as described in Lorwyn: First Light when messing with this variant. They are all great opportunities for aquatic campaigns that'll give players a chance to speak and interact with intelligent yet cunning beings.

Frost Giant Of Evil Water

Amphibious Giants You Should Fear

If you want a more pronounced water giant to add to your campaign and don't want to rely on the generic frost giant, the Frost Giant of Evil Water provides a few key specifications you'll find useful. Betraying the order, these gians are gifted the ability to breathe underwater, and they're outfitted with items such as harpoons that help them hook and drag creatures into the sea.

They're generally evil and can make a great disruptor in an aquatic campaign, especially by placing one in the deep underwater or having one drag the party down to the depths. They're also not quite as strong as Frost Giants, so you can use them without fear of annihilating the party in one hit.

22 Merfolk

Under The Sea

Not every monster needs to be an enemy. If you want to have underwater civilizations, the basic Merfolk works perfectly to be the 'commoner' of your story, though a group of them can still be a challenge for low-level characters.

They also have a CR six variant, which allows them to hit harder and have access to some spells that relate to water, such as Control Water, so they don't need to be just weak minions or commoners during the adventure.

21 Kuo-Toa

With Weird Gods

Kuo-Toas are small fish-like amphibious humanoids that used to live in coastal areas, but human settlements drove them deeper into the ground. Because of this, your party is likely to find them in isolated coastal caves or underground bodies of water.

With their low stats and CR, these creatures are not a real challenge in small numbers. But their cult-like behavior and their different variants make them a real threat even to mid-level parties at higher numbers, especially now that we have more options between the low-CR Kuo-Toa and the high-CR Archpriest.

20 Bullywug

From The Swamp To The Sea

Though Bullywugs are officially related to swamp areas, humanoid frogs easily fit aquatic adventures, so you have an interesting option here with the Bullywug Warrior and the sage variant for a stronger monster.

They're Fey and thus escape from conditions that affect humanoids. The sage, in particular, is relatively strong thanks to their free use of Ray of Sickness. They can also make a weapon attack on the same turn.

19 Sahuagin

Sea Raiders

A perfect option for lower levels is the Sahuagin. These underwater raiders can be quite ruthless and command sharks to do their bidding, so you can add them as minions for the battles to come. They also have an advantage against people who are hurt, so they get deadlier as the fight goes on.

They can go to the surface, too, so they're not exclusively underwater (the sharks are, though). They will need to submerge after a few hours out of the water, but your players will probably not worry about their well-being.

18 Harpy

Beware The Song

Sailors tell legends about the irresistible songs of the harpies and the many seafarers who have succumbed to a grim end once the singing starts. Harpies are iconic monsters in many works of fiction, and they make for challenging and memorable encounters in your D&D game.

Although they only have a CR of one, if your players fail to resist their luring song, an encounter with harpies can become extremely dangerous. Harpies are great monsters for low to mid-level parties, but with some adjustments, they can even be used against higher-level characters. For an even more aquatic flair, try homebrewing sirens from the harpy stat block; just replace their flying speed with swimming speed.