This week’s encounter is Aquatic Ambush, a standalone one-shot that you can drop into any D&D campaign while the characters are traveling on or underneath the water. From the open ocean to a lightless Underdark lake, these encounters are easily adaptable to any aquatic environment. Best of all, you can scale Aquatic Ambush to any tier of D&D play because of its simple premise: while traveling, the characters are assailed by amphibious attackers!

This encounter pairs perfectly with D&D adventures that require the characters to travel across dangerous waters like Frozen Sick (an adventure from Explorer’s Guide to Wildemount that’s one of our free Quarantine Resources), Out of the Abyss, or Ghosts of Saltmarsh. You can also drop this encounter into your homebrew campaign to stall your players when they decide to do something totally unexpected, like traveling halfway across the continent on a whim or even just going to a town that you hadn’t prepared yet.

Scaling this Encounter

This encounter can be scaled to suit characters of any level. Each version of this encounter is suitable for each of the four tiers of D&D play—optimized for play at 2nd, 5th, 11th, and 17th level. This is just the beginning, though. You can use the D&D Beyond Encounter Builder to further tailor this encounter to suit your party’s size and strength. If you’re playing with parties that are at the strong end of their tier, err on the side of increasing the numbers of the creatures in that tier, rather than pitting them against a higher-tier encounter.

For example, if you have a party of four 4th-level characters, you’re more likely to have a balanced fight if you increase the number of enemies in the tier 1 encounter. It might make more sense to reduce the number of monsters in a tier 2 encounter, but it’s easier to add power to a weaker encounter than remove power from a stronger one.

Why is tier 1 optimized for 2nd level, and not the lowest level of the tier (that is, 1st level) like the other tiers? Level 1 is a great starting point for new D&D players, but raising the base level of these encounters to 2nd level creates more opportunity to incorporate a diverse array of monsters and traps, rather than relying on the usual suspects for 1st-level adventures.

Combat Encounter: Aquatic Ambush

This encounter is suitable for characters of any level, if you follow the scaling guidelines.

While sailing down a river, through a swamp, across a lake, over the ocean, or over any other sort of large body of water, the characters are attacked by territorial creatures. The adventurers will have to figure out a way to fight when the terrain itself is fighting against them!

Campaign Tip: Who Needs a Boat?

Most people need boats to cross large bodies of water. The numerous boats presented in Ghosts of Saltmarsh are more than enough to provide your adventurers with a suitable vessel to get from one aquatic adventure to another. As such, the tier 1 and tier 2 encounters assume that the characters are traveling on a boat.

The tier 3 and tier 4 encounters, however, assume that the characters aren’t traveling across water on a boat. Once characters have reached this level, they don’t need simple vessels to cross the water. Instead, they’re traveling over the water via flight orwater walking, or under the water, perhaps using water breathing or even a legendary Apparatus of Kwalish.

Tier 1 Encounter: Bullywug Bullies

Read or paraphrase the following:

You slide through brackish waters in a small skiff, the smell of rotting vegetation thick in the humid air around you. A chorus of croaking frogs echoes all around you. Suddenly, the water around your boat starts to bubble! Slimy, frog-like humanoids with stone-tipped spears emerge, croaking in anger!

These six bullywugs croak angrily at the characters in their native, ranine language. This land is their territory, and the characters are passing dangerously close to their half-submerged stronghold. The bullywugs want to either drive the characters away to gain favor with their liege-lord, or to kill them and be hailed as heroes. They would rather avoid combat because a fight will most likely result in their own deaths.

Characters that make a successful DC 15 Wisdom (Insight) check determine that the bullywugs are puffing themselves up in an attempt to seem big and scare the characters off. If the characters divert their ship’s course and try to avoid the territorial bullywugs, they stop their attack.

Tactics. The bullywugs attack by swimming towards the characters’ boat, popping their heads out of the water, and then diving back beneath the surface. If any character dives into the water to pursue them, the bullywugs swarm that character and try to grapple them, then pull them down to the bottom to drown them.

Scaling this Encounter. If your characters are handling the bullywugs’ tactics easily, add a giant frog to this encounter on initiative count 20 of the third turn of combat. The bullywugs command it to attack, and it emerges from beneath the characters’ boat, capsizing it and spilling the characters into the water.

Tier 2 Encounter: Elemental Evils

Read or paraphrase the following:

The air turns cold as you sail across open water. Floating patches of ice begin to drift up from the beneath the water, surrounding your ship. Then, the ice begins to move! Crude, howling faces appear in the ice shards as they liquefy and swim towards your vessel.

This patch of water hides a tiny rift to the icy elemental plane known as the Frostfell. Because of this, these two water elementals are partially frozen, reducing their walking speed to 10 feet and their swimming speed to 70 feet. These peaceful elementals were attacked by ice mephits that passed through the Frostfell portal. The mephits accidentally merged with the elementals’ bodies, driving mephit and elemental alike mad with pain and torment. They savagely attack any living creature they can see.

Environment. Any creature besides the water elementals that fall into the freezing water take 3 (1d6) cold damage at the start of each of its turns.

Tactics. The water elementals rush onto the characters’ boat and try to use their Whelm action to engulf the characters. Once a creature has been grappled by the elemental in this way, it tries to leap off the boat into the frigid water and drown or freeze them to death by swimming to the bottom.

Mephits. When a water elemental dies, it bursts in a hail of sleet and ice shards, and four ice mephits surge from its remains. These mephits are still crazed from their time spent inside the elementals, and try to kill any living creatures in sight.

Tier 3 Encounter: Maw of the Marid

While traveling across the water, you spy a ship under attack! The vessel is cracked in two like an egg, and its two halves are sinking fast. A pillar of spiraling water rages between the halves of the ship, and atop the pillar is a portly, green-skinned creature dressed in extravagant clothing with a fish-like head and fins along its back.  

This fish-like being is an aquatic genie known as a marid. It is trying to get revenge on a wizard that trapped it in a conch shell, and is slowly but surely eliminating that wizards’ apprentices in an attempt to learn of his whereabouts. The marid just killed one of these apprentices, and her vessel is sinking to the ocean floor. The marid was about to leave, but then the characters arrived. The marid, thinking that they are allies of the wizard, bellows a command in the Aquan language. “Reinforcements, eh? I said I’d leave none alive, so by Olhydra I’ll leave none alive! Get ‘em!”

A group of five water weirds emerge from the water and attack. They have the following additional action, so that they can attack flying characters:

Water Jet. The water weird magically shoots water in a 60-foot line that is 5 feet wide. Each creature in that line must make a DC 13 Dexterity saving throw. On a failed save, a target takes 14 (4d6) bludgeoning damage and, if it is Large or smaller, is pushed up to 10 feet away and is knocked prone. On a success, a target takes half the bludgeoning damage, but is neither pushed nor knocked prone.

Tactics. While the water weirds attack from the surface of the water, the marid casts invisibility and then flies towards the characters. It then attacks using its trident, or uses its Water Jet action if it can hit at least 3 characters with a single jet. If the characters are thrown into the water, it uses control water (using the Whirlpool option) to catch as many of them as possible.

Setting the Record Straight. Of course, the marid is mistaken. The characters aren’t reinforcements sent by the target of its vengeance. If the characters can speak to him in Aquan, they can calm his wrath by explaining that they don’t know what’s going on and by making a successful DC 15 Charisma (Persuasion) check. If they don’t speak Aquan, they can halt his attack by surrendering or making a successful DC 25 Charisma (Persuasion) check to nonverbally show that this isn’t their fight. The marid then casts tongues on itself and tries to figure out what the characters are doing here. It lets them go free if they explain that they have nothing to do with his quest for revenge—or better yet, the marid tries to rope them into its quest for vengeance.

Rules Tip: Falling Prone While Flying

If a creature falls prone while flying, it falls to the ground at a rate of 500 feet per round. It takes falling damage as usual, even if it hits water.

Tier 4 Encounter: Dawn of the Dragon Turtle

Read or paraphrase the following:

The placid surface of the water explodes in a torrent of wrathful waves and seafoam as a massive turtle breaches out. Strapped to its enormous shell is a palatial saddle made of solid gold, with a roof and four different rooms. Inside this mighty saddle is a veritable army of shark-like sahuagin, all roaring and waving their spears in a display of aggression.

This army of 24 sahuagin are led by a triumvirate of three mighty sahuagin barons. This army is supported by six sahuagin priestesses. Their miniature palace is strapped to the back of a dragon turtle named Aguazath, who agreed to work with the sahuagin in exchange for plentiful food—preferably the tender flesh of their humanoid captives.

Flying Palace. The sahuagin have ample supplies of spears to throw at their enemies, but it’s possible that the characters can simply fly out of reach of their spears. That’s why the sahuagin barons’ mighty dragon turtle saddle has been equipped with four magic items similar to wings of flying which allow the entire structure to fly through the air with a flying speed of 60 feet. It can fly in this way for a total of 1 hour each day, and the sahuagin need to unstrap the twelve straps that attach the saddle to the dragon turtle in order to let it fly. As long as there are at least twelve sahuagin on board, unbuckling the secure straps only takes a single action.

Sahuagin Tactics. The sahuagin throw spears at their enemies so long as they’re in range. However, the rank-and-file soldiers of this sahuagin warband are eager to risk death if it means impressing their barons in this life, or their goddess Sekolah in the next. If they can, they take a running start and leap up to 13 feet away to grab onto a character and try to grapple them, hoping to plunge them into the water.

Meanwhile, the priestesses start by conjuring their spiritual weapons. Then on future turns, they blast distant foes with guiding bolt and attack with their spiritual weapons. If necessary, they use hold person to halt flying foes, and try to keep their barons alive with mass healing word.

The sahuagin barons have a large supply of tridents to throw, and hurl them at the foes until they’re in melee range. They simply try to do as much damage as possible.

Dragon Turtle Tactics. The dragon turtle prefers to let the sahuagin do as much of the fighting as they can. Once a character is in the water, however, the turtle tries to tear them apart with its melee attacks. It uses its Steam Breath if it can catch at least two characters in its area. The dragon turtle flees if the sahuagin barons are killed, and begs for its life in Aquan or Draconic if the characters pursue it and try to punish it.  

Treasure

Consult the random treasure tables in chapter 7 of the Dungeon Master’s Guide to determine the amount of treasure these creatures are carrying. Choose the Individual Treasure table that suits the challenge rating the encountered creatures. Then roll once for each creature that would logically carry treasure on its person.

For example, after the characters defeat the bandits in the tier 1 encounter: Bullywug Bullies, you would roll six times on the Individual Treasure: Challenge 0–4 table. Even though there are seven monsters with a CR of 0 to 4 in this encounter, one of them is a giant toad that serves the bullywugs—it wouldn’t logically carry any treasure.

Other Travel Encounters

This encounter is part of a mini-series of scalable encounters that you can drop in your party’s path as they’re traveling across your campaign setting—whether that’s the Sword Coast, Eberron, Wildemount, or a world of your own creation. Check out the other encounters in this mini-series in the Traveling Encounters tag!

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Did you like this encounter? Check out the encounters in the Encounter of the Week series. You can also pick up the adventures I've written on the DMs Guild, such as The Temple of Shattered Minds, a suspenseful eldritch mystery with a mind flayer villain. My most recent adventures are included in the Platinum Bestseller Encounters in Avernus, a collection of over 60 unique encounters created by the Guild Adepts, which can be used to enhance your campaign in Avernus or elsewhere in the Nine Hells. Also check out the Platinum Bestseller Tactical Maps: Adventure Atlas, a collection of 88 unique encounters created by the Guild Adepts, which can be paired with the beautiful poster battlemaps in Tactical Maps Reincarnated.

James Haeck is the lead writer for D&D Beyond, the co-author of Waterdeep: Dragon Heist, Baldur's Gate: Descent into Avernus, and the Critical Role Explorer's Guide to Wildemount, a member of the Guild Adepts, and a freelance writer for Wizards of the Coast, the D&D Adventurers League, and other RPG companies. He lives in Seattle, Washington with his fiancée Hannah and their animal companions Mei and Marzipan. You can find him wasting time on Twitter at @jamesjhaeck.

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