Plesiosaurus

A plesiosaurus on the party's side is rare and almost always a druid's wild shape, a shoreline community's totem raised from a clutch, or a one-time gift from a sea hag the party has done a costly favor for.

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

If a fight is unavoidable, the plesiosaurus's job is to grapple and pull. Even without a written grapple action it is Large with a long neck, so call a Shove of 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 plesiosaurus is not a bruiser; it is an interrupter.

Name the plesiosaurus 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 rises under the rowboat and is gone before the party finishes their saving throw. CR 2, 68 HP, AC 13, swim 40 feet, Stealth +4. It functions as an aquatic ambush predator. The Bite is +6 reaching 10 feet doing 2d6+4 piercing. Hold Breath means it can stay under for an hour.

Set up the ambush. Roll Stealth from underneath; the party should not see it coming. The plesiosaurus rises in the boat's wake, 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 should include a clear path back to deep water. If the boat has a sail, the long neck snaps the mast.

Run it as a complication, not a duel. The fight is more interesting if the party is doing something else: rowing for shore, holding 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 to chase it lose action economy; the plesiosaur is at home and they are not.

If the plesiosaurus drops below 34 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. The encounter is a story about the sea, not 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."