I know its not fair to judge a book against its predecessor, especially when it's predecessor was The Girl on the Train; however, it's hard to be impressed by Into the Water knowing what Hawkins is capable of writing.
Into the Water is fine, it's not captivating in anyway but its an easy, candy read. Similar to The Girl on the Train, the narrating voice shifts between chapters, but Into the Water doesn't shift as well. There are so many character shifts that it gets really confusing.
I enjoyed the main story line, I enjoyed the witchy folklore of the small town in which the story takes place. I also enjoyed the characters of Jules and Lena.
I wouldn't say that this is a skip, though I would suggest moving it down your to-read list.