Perhaps some of the chaotic good inherent in their storm giant forms manages to cling to the water yai. For though they are still chaotic evil creatures of great power, the Bestiary 3 notes: “Water yai tend to be easily distracted by beautiful treasures, and despite their evil natures are prone to acts of unexpected frivolity.” Plus, when your form is that of a Huge CR 18 three-eyed giant, you can afford to not crush every or dominate every humanoid you see.
But more likely, life above the surface just holds little interest for water yai. They have sunken galleons of full of treasure and lost aquatic elven cities to explore—provided they are left alone. Water yai as described in the Bestiary 3 are likely highly territorial below the waves—they may “have no real longing to rule or infiltrate societies,” but they also don’t want anyone messing with their precious treasure. So an isolated water yai left undisturbed might remain a capricious neighbor without being a true threat. But a water yai whose playgrounds are disturbed by treasure hunters or pearl divers, who dwell beneath shipping lanes too prosperous to ignore, or who face competition from other great undersea powers are likely to be forces of terrible envy, rage, and destruction.
The water yai Draga is insane. She rules a kingdom of her own imagination, sparsely populated only by the phantasms and conjured creatures called up by her undine illusionist and summoner granddaughters. The spellcasters’ noble efforts to keep their grandmother quiescent become vastly more difficult when human craftermages discover a font of phlogiston not far from the isolated spit of land Draga calls home. No amount of illusion magic can hide the drilling platform being constructed on the horizon, and the blight on her kingdom’s skyline is likely to rouse Draga to truly terrific rage.
Wise Aunt Belar was happy to dwell beneath the waves, dressed head to fishy tail in the pearls and silks looted from a hundred sunken junks. Then one moonless night an elemental-power steamship ghosted into her cove to meet a crew of smugglers. Wise Aunt Belar watched with interest the clandestine exchange of opium for gold and firearms that took place. In a manner of days she subdued the smugglers, and now she runs a massive drug-running operation, with merfolk and awakened octopus slaves shipping product as fiendish saltwater merrow overseers look on. Meanwhile, her cove is now guarded by cannon works—and cannon golems—that any seafaring nation would envy.
A cursed sword forged by an efreeti can only be destroyed by the acid from the third eye of a water yai. The nearest such oni, a corpulent beast named Jasper, desires a wife, and securing a betrothal could go a long way toward securing his cooperation. His great bulk, evil nature, watery home, and rather eclectic tastes—he once attempted to woo a swan maiden, a vishkanya assassin, and a foo lion at the same time—make matchmaking a challenge.
—Pathfinder Bestiary 3 212