CR- Variable, Minimum 5, recommended 10
It's been a hot minute since the last encounter culture article, so let's cool off with a new one. Ships and boats are classic set pieces in most fantasy games, but many of their fights tend to happen the same way. Enemies try to climb up the boat, or board from an enemy vessel, and they're fought on the deck and stop and killed as they board. This encounter gives the opponents a leg up and gives the players a fair bit more to worry about. Magican Sahuagin.
Basic premise: Sahuagin spellcasters use magic to teleport aboard the enemy ship, and damage it enough to require urgent repairs.
The breakdown: This encounter takes standard Sahuagin and gives them 2 levels of Wizard. They are not given generalist wizard however, instead taking the school of conjuration and the subschool of teleportation. This grants them the all important shift ability. Allowing them to teleport up to 5 ft as a swift action 3+int times per day. An average Sahuagin gets 5 daily uses of this, but they'll mostly only need one.
The Sahugagin break into teams, some attacking from above by climbing the sides of the ship and/or teleporting aboard duck depending on the height. While a second team bores holes in the underside of the ship. The shift ability requires line of sight and so the Sahuagin are best served by making these holes so they can look inside and then teleport below deck. This also creates the problem of many small holes in the ship which could sink the vessel if not repaired.
In general a player should be allowed to patch an adjacent hole as a move action. The fight should ideally be a tough mix of repairing holes, and fighting Sahuagin who can teleport about as a swift action. Remember that Sahuagin's blindsense allows them to hop to the other side of walls as long as they can perceive what's on the other side. (Though blindsense is very poorly defined in terms of seeing objects rather than creatures).
With a few spells available the Sahuagin would be best served with a mage armor cast before hand. As far as offensive magic a good burning hands or two on whatever ship parts are dry would not be remiss.
For extra style, these holes and the weight of the ship pushing down could make arcing jets of water cross the lower decks, which could be used to describe things with more verve. Imagine an ice spell passing through such a stream making ribbons of ice in the air.
Tale of the tape: A sahugain given 2 levels of wizard increases to cr 3, and this plan needs a very minimum of 2 Sahuagin, which would be a cr 5 encounter, but pumping those numbers up a bit is not a bad idea. Since the ship itself is in as much danger as the players it's totally fine to add enough that individual foes are fairly trivial. Even if the players can always hit and kill them and not be hit by their attacks it can still be very dangerous having 32 angry magical fish people aboard the ship drilling holes and lighting fires. Plus the usual solution to masses of enemies, area of effect attacks, is a far less appealing idea on ones own boat in tight confines. And of course if a dozen or so of the angry fishfolk all fire their magic missiles at the same foe it could still be rather lethal.
Variations: The easiest ways to change up this encounter is by swapping Sahuagin for another type of creature, by simply changing their spell selection, or by making the ship flood/burn faster or slower. But beyond those simple variations this could also be applied to other ships. Air ships being attacked by Sahuagin with flight magic, or even spaceships being assaulted by crews of teleporting boarders. This could also be done off of a ship using wooden houses, forts, or castles to create a very unconventional siege that focuses on keeping the enemy inside the keep and not letting them reach and open the gates to let in the rest of their army.
Hopefully this encounter has helped you spice up your next shipboard battle. Until next time, take care and have fun!