In an underwater world, two introverted nerds begin a correspondence and fall in love, then disappear in a tragic accident. A year later, their respective siblings share letters to deal with their grief and perhaps find out what happened.

The worldbuilding here is fascinating, since most of the planet is covered in water and there is very little dry land, so people mostly live on ships or artificial islands - and the main character, E., actually lives in an underwater house. Also, E. refuses to use her full name because she thinks it's embarrassing.... but honestly, compared to her siblings, it's not even that bad.

I liked how this all started out with letters between two people and then gradually grew to include letters and documents from others, as well as poems and chapters from books. The whole story is written in documents and occasionally transcripts of conversations, with no spoken dialogue. 

The characters are also amazing. Henerey is a Puppy, E.'s chronic anxiety is presented in a very relatable way, and I love how Sophy and Vyerin form a friendship through the letters.

Of course, there is a big world-shaking mystery as well as a family secret, because apparently nobody in this family knows how to communicate with each other. I loved the unfolding mystery, but this book is definitely a first in a series, because it kind of ends in the middle of interesting action.