As the title suggests, this short novel focuses on water and its denizens, in particular two species, selkies - shapeshifters which turn from human to seal - and merrows - evil fish-men similar to mermaids, with the jaws of sharks and poison-laced claws. An oil spill at sea has displaced both merrows and a magically impeded selkie to Sunnydale, creating a crisis. The juvenile selkie - loved by some, feared by others - is difficult to understand in its language and motives, and a mystery is established over whether or not it can be trusted. Meanwhile, attempts are made to return it to the sea by re-enchanting its seal coat. But semi-intelligent merrows are taking Sunnydale by storm, slaughtering humans and vampires alike, and singling out magic-users. With a turf war brewing between the merrows and the town's resident vampires, the scene is set for a three-way showdown with Buffy and friends at the centre.
This novel doesn't break continuity as badly as most, although Giles is too friendly with Angel, and the idea of Giles using magic is unusual. It is located towards the end of season three, with Buffy, Angel, Willow, Xander, Giles, Oz and Cordelia all featuring, but after Cordelia had broken up with Xander. In addition the book introduces Dr Lee, a fanatically anti-selkie biologist, and the delightfully impish selkie Ariel.
The book very effectively reproduces the humour of the TV series, especially the characters' sarcasm and retorts to each other. Like most TV episodes, it addresses human emotional issues against a backdrop of supernatural events, in this case exploring forgiveness after separation. Some themes in the book are perhaps a little too disturbing - the idea of vampires surviving dismemberment in particular.
On the other hand, with the exception of the portrayal of Cordelia, the book lacks emotional depth. Buffy's supposed phobia of water is treated in a stereotypically one-dimensional way. The merrows do not stand out as villains, basically appearing as sea-borne vampires; they compare unfavourably with most of the TV and novel villains. Their motives are disappointingly straightforward, and even their appearance is not well explained, oscillating between human-like mermaids and shark-headed fishmen. The story never really takes off; the two storylines - the merrow/vampire war and the selkie/Dr Lee storyline - coexist rather than melding, their origin in the oil spill being the only real link. Although the issue of the selkie's motive provides some mystery, the book is largely lacking in suspense compared, say, to the handling of werewolves in the TV series; it would have been strengthened, for instance, if the merrow attacks were initially blamed on the selkie child or her family, creating the kind of suspense found in "Phases", or if the merrows and selkies were given some kind of historic feud or connection. It's not clear why it was thought necessary to prevent the vampire-merrow turf war. Nor was it practical, without allying with one side or the other - an option which was glaringly absent from consideration (Buffy working with Spike or Angelus to defeat the merrows would have made for an interesting twist). The resolution of the merrow situation is similarly lacking, with their actions basically unexplained. Dr Lee is a one-dimensional character; while he introduces the important emotional backdrop of inability to move on, he does so in an unbelievably simplistic way, again lacking in explanation or resolution.
Overall I felt this book has its strong points, but doesn't really live up to the Buffy franchise. It's a frustrating few improvements off being a really good story.