Undercurrent was a decent novel, but I didn't find it as engaging as 'Siren'.
Still, it's nice to read a book about sirens. They're not the most popular supernatural creature in the YA Supernatural literary pile, so there's a sense of novelty and creativity in the books that is harder to obtain in other, more used supernatural creatures.
The ending was fantastic - things are far from being the perfectly wrapped package and keep me interested in reading the final book of the series.
The one thing that would have made this much more interesting would be if Parker was a male siren. I would have enjoyed that twist more than the predictable one that occurred.