One final sea entry for you: the seaweed leshy!
We haven’t had a leshy in a while, so those who don’t have the Bestiary 3 should know that a leshy is a nature spirit summoned by a fey or plant creature druid to inhabit special plant bodies—like a homunculus, essentially…but still tied closely enough to nature to be a plant, not a construct.
Seaweed leshys are the most powerful ones so far described, weighing in at CR 3 and with an overproud nature that lends them to mocking their freshwater brethren. Like most leshys they are stealthy (constant pass without trace, tree shape into Small Seaweed) and can defend themselves in small ways (entangle 1/day, a bludgeoning, blinding water jet). But their real boon to adventurers will likely be the water breathing-granting air cysts they grow—which might mean life or death (or at least a way of bypassing a troublesome room) in an underground dungeon.
A bed of rare, blue-pearl-producing oysters is tended by a kelp walker (a kind of treant-like seaweed). When the often-restless plant creature goes on walkabout, a patch of seaweed leshys stand guard instead.
Creations of a nereid druid, seaweed leshys guard a half-sunken island complex. Their knowledge and air cysts would be useful in negotiating a tricky section of the cave system, but since the adventurers are bearing a token from a freshwater nixie, the supercilious seaweed leshys see no reason to help what they see as the thralls of a positively provincial fey.
The uncle of an adventurer dies, leaving behind as his legacy a house high in the mountains incongruously filled with nautical artifacts from his days as a ship’s captain. In among the treasures is a flask with a preserved seaweed leshy inside; if watered, the leshy rejuvenates. It then sets about alternately encouraging the adventurer to finish her relative’s last mission (which failure caused the uncle to settle far, far from shore) and trying to “defend” the house from intruders (including the adventurer herself!) until a successful Heal or Knowledge (nature) check is performed to calm it.
—Pathfinder Bestiary 3 180