The Drowned Woods is a young adult fantasy story. There is a very overpowered main character, a "water diviner", someone with the ability to control water. Although most of the time she seems to control the temperature of water, which is something completely different, but whatever. She has the usual sad story of someone coming from humble origins and then having bad stuff happen to her, usually because of rich and powerful folk, but in the end all of these events strengthen her enough to handle the weight of the consequences of the plot.
Emily Lloyd-Jones skillfully combines Celtic mythology with Avatar-like magic (Avatar with the bending, not the blue people) and a heist plot in a fun enough story. Which means it has the typical heist structure: assemble the team, do a lot of flashbacks so we get to know the characters, show how their skills come together, start the mission, the twist, the epic finale.
It's not like I didn't like the book, I just couldn't possibly love it. The reveal at the end is pretty interesting, but not unexpected. Most of the time our heroine is either handling everything, has someone help her in the 11th hour to handle everything or she is (or the reader is) misreading the situation, which means even what could be considered defeat can be retconned later into a complete victory.
Bottom line: easy to read, reasonably interesting, but not very captivating.