One last entry for Sea Week, and it’s the most iconic one of all: the sea serpent.
Oddly, sea serpents didn’t (must…resist…temptation…to…) make a big splash (nooooooo!) in most editions of D&D/AD&D. It’s just one of those weird gaps that pops up in D&D from time to time—if Gygax didn’t put it them in the original Monster Manual, some mythological beasts just never get noticed, no matter how glaringly obvious they are.
In Pathfinder though, sea serpents are exactly where they should be in the core Bestiary, ready to capsize, constrict, and devour your PCs on their next ocean voyage. Richard Pett covers most of what you’d want to know about sea serpents in Mystery Monsters Revisited, including offering plenty of notable examples of the breed from all around Golarion and a society dedicated to hunting them. Since he’s covered that, I’ll just add that the key part of sea serpents is their elusiveness. Mid- to high-level players are used to having information at their disposal—they have the divination magic, the Perception ranks, and the muscle to get the answers they want. Sea serpents defy this expectation—they are hard to find, and once found they are hard to kill, much preferring to flee than face death at the hands of air-breathers.
Finally, of course, the sea serpents in the Bestiary are just average specimens. With templates and/or advancement, they get much bigger…
The northwest corner of Wesleyan’s Atlas, one of the only reliable maps to the Arborean Cape, is festooned with sea serpents. These are not fanciful illustrations. Due to the rich fisheries in that area, sea serpents teem in the chilly waters. Most are serpents, but a few are fatter, blubbery fish creatures that trade their constrictive abilities for a buffeting waterspout.
A courier bears a ring that establishes the authenticity of a once-exiled dauphin’s claim to the throne. Unfortunately, the courier herself lies—or her bones do, anyway—in the belly of a sea serpent. Her identical twin is determined to rescue her sister’s body and restore the prince to his throne, but she needs capable help.
A heavily scarred and tattooed man seeks adventurers to settle an old score. He says a sea serpent took one of his master’s legs, and his master wants the serpent found and destroyed. All is, of course, not what it seems. The tattoos and scars actually hide that the man is a gillman, and his master did indeed lose a leg to a sea serpent…but being a kraken, it had several more to fall back on. Mostly it is only the kraken’s pride that is hurt.
—Pathfinder Bestiary 244