Deep water - Laura Anne Gilman

Joining a volunteer rescue team to clean up an oil spill which threatens the coast, Willow discovers an unexpected victim on the shore: an oil-stained young girl whose legs are tangled in soft leather skin. She's a selkie; a shape-shifting seal girl, and she can't return to the sea until her coat is cleaned. Buffy and the Slayerettes take responsibility for the bewildered girl, whom they call Ariel. But Buffy can't shake her innate suspicions of the creature, whose nature, like the ocean, is ever-changing. For everyone's sake, Ariel must be returned to her own environment soon.
Unfortunately the spill has roused more than a selkie. Merrows look much like traditional mermaids - except they drink blood. Disturbed by the pollution, they've come to shore, which triggers the territorial instincts of Sunnydale's resident vampires. As if things weren't complicated enough...