Yeah, but THIS Sea Serpent...

d12
1. Face splits open to reveal cerebral tissues pulsating w/psychic potency, places most intelligent human present under mind-control to act as mouthpiece, issues inscrutable demands (see subtable, below)
2. Exceptionally elusive: surges in from below, cripples isolated ships w/blunt force just to watch them sink, becomes visible momentarily during attacks but eyewitness descriptions vary wildly, immune to spells of detection
3. Begins laughing upon sighting lone vessel, a high-pitched hyena-like cackle, needle toothed fish mouth twisted into hideous smile
4. Rows of extensible oversize flippers allow spectacular leaping/gliding, like right through sails/rigging, onto deck crowded w/tasty humans
5. Head of a wizard, pointy hat w/chinstrap, gesticulates w/tentacle beard, casts variety of maritime spells
6. Escaped prisoner of armored dolphin warriors (in hot pursuit), wants to hide out in ship's hold
7. Giant body of indeterminate length, seems to go on for miles, can throw enough mighty coils to crush a fleet in one go
8. Currently recruiting soldiers for war w/mermen to satisfy personal vendetta, tends domesticated mollusks that produce water-breathing enzyme
9. Announces presence, grants time to board lifeboats before destroying ship
10. Shoots streams of super-heated water, internal boiler-organ explodes violently if slain
11. Commands army of barracuda, on the run from superior forces of intelligent cephalopods
12.  Hopelessly in love w/titanic ancient kraken that compulsively guards treasure-laden graveyard of sunken vessels

Psychic Sea Serpent: Inscrutable Demands
d12
1-2. Just wants to eat a single adventurer, the rest may go on about their business
3-4. Insists all golden treasure be thrown overboard
5-6. Requires detailed information re: regional navies
7-8. Needs human breeding couple, never mind why
9-10. Must have large quantity of lethal poison
11-12. Craves surface delicacies like cake, booze