I too just finished reading and I believe it was Andy. Like you, I was stuck between accepting this cliffhanger while loving the whole “leave it to the reader” feeling, and getting pretty upset that I had no conclusion. Re-reading the prologue helped me conclude it was Andy. I can’t help but think that the prologue was describing the Marsh in the perspective of the lady who owned the Monkey where it describes her living waaaaaay in the back of the Marsh who quietly witnesses everything, and according to the prologue, she ends up witnessing the murder of Sarah-Anne. I also believe it’s in her perspective because throughout the book they referenced the lady with the monkey's essentially being invisible, and the only person who knew she was living farther back was Andy. The prologue also eludes to the murder by drowning. Maybe Andy drowned Sarah for revenge? Maybe Nile?

Now, I feel like the book made it pretty clear that Nile and Clay definitely got on the boat and left for work…I was also a little skeptic about it, but if Nile went on the boat and knew Andy killed Sarah, it explains why he felt soooo bad and took the kids in.

Kay also referenced Andy’s work boots a few times then later mentioned finding the boot print in the shed.

It was either Andy or Nile…but I’m leaning more towards Andy. Sorry my notes are all over the place!