70% gaslighting, 20% author's misogyny, 10% squid things 
I ignored the plethora of negative reviews, so truly I only have myself to blame for even reading this. 
The author has a lot of hangups regarding pretty girls and going to the gym, and it truly bleeds through so much in Those We Drown. I should have kept count of how many times Liv talks shit about the Sirens in her head or out loud trashes them for the way they look, and their jobs. There's also a weird moment when some of the characters are making fun of another for going to the gym, and say something along the lines of "I'd rather eat oreos..etc etc". Did you know you can eat Oreos and go to the gym? Fun fact for you.
Liv finds out and susses out that something is afoot early on, and the rest of the story is everyone just telling her she is crazy. The first few times were tense and then the 38th time I was like. Girl. Please. Stop. For someone supposedly so smart why are you like this? 
The characters are extremely one dimensional. I couldn't tell you anything about them other then the one note that is their personalities. Liv's only characteristic was her obsession with Will. Seriously.
Constantine was such a weirdly inconsistent character, I truly still don't understand half of his and Liv's interactions. It almost felt like the author was going to take his character a different way and changed her mind halfway through. I didn't read them as having any chemistry at all. It was...dull.
Oh and there is zero diversity. (What about Raj you say? Does anything about him even read BIPOC I say!?) 
The book just kept going, on and on and on. It truly felt like it could have been half the length. I kept waiting for action, or other things to happen. Instead people kept disappearing or getting murdered and no one seemed to care about anything, except convincing Liv she was imagining it. 
I did like: the writing style was the only reason I kept reading, I was engaged and hoping it would get better, or have twists and turns. (It did not...) I also liked the imagery, it was very creepy and had great descriptions. 
The ending was a poor attempt at a cliffhanger, but instead just had me feeling glad I could stop reading.  We already knew what was going on by the time the first few chapters, so why drag it on? It isn't even that it is a breadcrumb trail of hints and clues, but so heavy handed anyone could easily see what was going to happen. I really thought there was going to be a twist, because no way would a book remain so obvious and simple...I was wrong. 
A better ending that I thought was going to happen (again, there are ZERO twists or surprises) would have

been having Con actually be apart of the cult, and actually be the captain. He could have aged backward and that would have explained his accent, the way he plays the field, and his overall lack of willingness to believe Liv. They could have gotten on the lifeboat and then he could have turned to her and started laughing, and then she would have been trapped with no way to escape. Her fate would have been sealed.

Instead we are spoon-fed the ending that is given to us freely within the first part of the book.
One last note, there was such a emphasis on Liv's inability to swim and her fear of water, SO WHY DIDN'T WE GET ANY SWIMMING SCENES HMMM???!!
I would NOT recommend this book, nor would I read anything from the author again.