Plesiosaurus

A plesiosaurus on the party's side is rare and almost always a druid's wild shape, a shoreline community's totem animal raised from a clutch, or a one-time gift from a sea hag the party has done a costly favor for. Commit to the framing in the opening: the plesiosaur is wild, it tolerates riders, and it is not a horse with flippers.

Use it as transport. The 40-foot swim speed beats almost any sailing ship over short distances, and a plesiosaur can carry two or three Medium passengers on its back through open water. The long neck makes it a credible scout for what is actually under the surface; have the player ride forward and roll Perception (+3, passive 13) for the plesiosaur while their PC squints at the horizon. AC 13 and 68 HP are enough to survive most random sea encounters but not enough to fight a sahuagin patrol; if real combat starts in the water, dive deep and wait it out.

If a fight is unavoidable, the plesiosaur's job is to grapple and pull. Even without a written grapple action it is Large with a long neck, so as a GM call you can let it Shove a Medium creature off a deck or drag a swimming opponent into the deep on a contested Athletics check. Do this once, then surface. A plesiosaur is not a bruiser; it is an interrupter.

Name the plesiosaur and let the players see it eat fish between encounters. Big animals on the party's side are loved when the camera spends thirty seconds on them doing nothing dramatic.

A plesiosaurus is the long-necked terror that comes up under the rowboat and is gone again before the party finishes their saving throw. With a 40-foot swim, AC 13, 68 HP, and Stealth +4 it functions as an aquatic ambush predator. Run it as a kraken's smaller cousin: dangerous when it surprises you, much less dangerous once everyone is on dry land and someone has a longbow.

Set up the ambush. Roll Stealth from underneath; the party should not see it coming. The plesiosaurus rises in the wake of the boat, takes a bite, and slides back into the water before the rogue can react. With a 20-foot walk speed it is helpless on shore and knows it, so every action plan should include a clear path back into deep water. If the boat has a sail the GM should already be thinking about how the long neck snaps a mast.

Run it as a complication, not a duel. The fight is much more interesting if the party is doing something else at the same time: rowing for shore, holding a sail against a storm, keeping a captive alive. The plesiosaur takes its bite, dives, and comes back two rounds later from the opposite side. Players who dive in to chase it lose their action economy entirely; the plesiosaur is at home and they are not. Use the 40-foot swim to keep range from the swimmer who actually has a weapon.

If the plesiosaurus drops below half HP it leaves. Beasts do not fight to the death, and a wounded plesiosaur sinks into the cold and finds a different shoal of fish. Do not feel obliged to kill the party with it; the encounter is a story about the sea, not about the body count.

Describe the wake first. Two long ripples in still water, closing on the boat. Roll initiative when the players say "what was that."