Source: Bestiary 4, pg(s). 208
Oceanids are sea-bound fey, close cousins of dryads and nymphs. Although they can survive for short periods in freshwater lakes and rivers, their permanent home is the salty ocean. Their lower torso and legs take on the shape of a pillar of water while immersed in water, while on dry land, the water transforms to a pair of human legs. Oceanids occasionally befriend sea-going land creatures, especially if that creature appeals to the oceanid's extreme vanity. Their moods can change rapidly and with little warning, like the sea, and they are especially prone to anger.
Inner Sea-dwelling oceanids are famed for the help they bring to ships that have got lost and will, if the crew pays proper respect, lead the ship to safer waters; if offended, oceanids are as fickle as any fey, and will swim away leaving the crew to their fate. Unfortunately, some evil fey pretend to be helpful oceanids, using the goodwill sailors owe to oceanids to trick crews into taking their help but then betraying the ship at some critical juncture.