Buffy the Vampire Slayer: Deep Water

Shape Shifters
Willow's soft spot for critters finds her spending a cold winter morning along the coast as part of a volunteer rescue team, cleaning up an oil spill that has damaged the marine habitat. While climbing over some rocks, she discovers another unexpected victim of the spillage -- a selkie, a shape-shifting seal girl who won't be able to return to the sea until the oil is removed from her coat. Willow takes the creature back to the library so that Giles and the Slayerettes can help her to restore her magickal coat. However, though "Ariel," as the posse dubs her, is endearing in her innocence, Buffy can't quite shake her innate suspicions of the creature whose nature, like the ocean, is ever changing. Unfortunately, the spill has forced more than a selkie from the cold water. Merrows look very much like traditional mermaids -- with one important and fatal difference. As if things weren't complicated enough...

This novel is non-canon.