Listen up, adventurers. Don't go underwater. It's awful. And it doesn’t get any better as you go up in levels because that means the adventure just goes deeper, and deeper is always bad. What’s that? You have a race or class ability that lets you breathe water? Too bad you have nothing to handle the water pressure crushing you like a tin can. You have a magic item that gives you a swim speed? Great, you’re now almost as fast as the leviathan that will swallow you whole for acid and/or bludgeoning damage. You inherited a magical bathysphere? Awesome! The monster is so big it can smash through its hardness.
Case in point: the deep sea serpent.
How bad could it be, right? The sea serpent is CR 12. The deep sea serpent is…CR 19. 15 Hit Dice…vs. 23 Hit Dice. Tail slap +18…vs. tail slap +33. A possible 50 points of bludgeoning damage in the stomach vs. 81 points. Plus a charge attack that can take it 400 ft. in a single round.
And it’s not even smart! This probably won't even be the adventure finale! This is the monster you have to go through on your way to the bad guy.
Adventuring underwater never gets easier. Save yourself a lot of trouble and let the krakens or the aboleths or the whatevers win.
(As for GMs: Go nuts! Add bioluminescent lures, special attacks, templates, and fashion them after strange creatures like gulper ears and goblin sharks… Your players will totally love you for it. Honest.)
A ceratioidi ruler is also a powerful arcanist (see the Advanced Class Guide). Using her magical gift, she seeks to hasten her apotheosis into a Great Mother, hoping to gain the physical and mystical power of the Mother without losing her great intellect. What she didn't account for was the effect the experimental rite would have on her male counterpart. Now she is locked in a battle over her own body. If adventurers do not intervene in time, the male will bloat into a deep sea serpent and destroy all the ceratioidi matron has built.
Most of the Plane of Water is as hospitable as a shallow sea, but there are natural and unnatural zones of pressure beyond that of even the deepest Material Plane oceans. To protect their eggs from fell influences beyond the stars, certain vortex dragons hide their clutches in the Plane of Water, where their presence creates a gravity and pressure all its own. These clutches are guarded by deep sea serpents who recognize the eggs as the source of the comforting crushing sensation and guard them as their own.
To prevent the colonization and utter enslavement of Sheyaria by Taltos conquistadors, the elven nation of Wendaerin raised an entire fleet to save their dawn elf kin. The effort was monumental, with materiel transported through Weal Gates from other worlds and the entire island port of Bandwn held in a bubble 10 years out of time in order to finish the work on schedule. Now a fleet like none other in history sails forth. But before it clears the Bay of Orvall, not just one, but two deep sea serpents rise to meet the boats. If the monsters are not defeated, the rescue will be over before it has even begun.
—Pathfinder Bestiary 3 240
Sure, I own the Advanced Class Guide. Because credit card. Given my current level of work/life balance—the earliest I made it home this week was 10:15 PM—let’s all just pretend I’ve read it, shall we?
If you’re a D&D 3.0/3.5 fan, those vortex dragon eggs could easily belong to the Epic Level Handbook’s force dragons.