3.4/5 Goodreads; 3.8/5 Amazon, despite the poor proofreading. Original plot, believable characters
This is a very plausible scenario for what would actually happen if a severely injured merperson washed up on a U.S. beach. Ordinary people react in very human ways, from the discoverer determined not to be called a liar; to medical professionals at a loss for what their patient needs; to reporters eager for a great story, whether it be an amazing hoax or a scientific marvel; to rival military and diplomatic officials torn between eagerness to investigate and exploit this new people and fears of their possible numbers and plans.
Meanwhile Chris's adoptive family (a powerful businessman father and two young brothers) as well as other merfolk "on the surface" (i.e., living incognito among humans), debate what they can and should do -- including whether or not to wait for, and obey, the directives from the restrictive regime of the City under the sea. Once Chris awakens from his coma, things take a new turn among those in the know, despite the wary silence he maintains from his hospital bed, determined to avoid accidentally giving away any more secrets than his mere physical presence in scientists' hands, trapped in a cryptic combination of scaled tail and human-like lungs, reveals on its own. Telepathic communication with his own kind provides a vital connection with hope, but also at times frustration.
I was gripped from the beginning, eager to see how it all came out. Overall, I was pleased, though a few plot-threads have not yet been resolved, the most important being the identity of whoever initially attacked Chris at sea -- in other words, someone ill-intentioned is still at large, and in possession of their secret. Other elements are revealed gradually, such as the boys' pasts and the philosophical divisions among the merfolk, as well as the origins of their race and the goals of their City. Almost all the characters are depicted realistically, rather than as black-and-white saints and devils, from impatient teens to ambitious officers. The book ends with a suggestion that a larger-scale "coming out" is inevitable within, at most, decades. I will be watching for future releases, and checking out Undercurrent on the author's website. (ETA: Oh, I misunderstood: Undercurrent — A Merfolk Myth has not yet been completed.)
Regarding extras, there is what's called a "bonus chapter", titled "Moments", but it's just a handful of very brief flashbacks. To me, they're TOO brief, affording nothing more than tantalizing, but rarely satisfying, glimpses. N.b., they're posted online, too, along with several more.
Now, as for the editing, I didn't catch any internal inconsistencies, but there are too many paragraphs with awkward or downright wrong syntax and/or word-choice, plus a couple missing words, and should-be-apostrophes that are facing the wrong way at the beginning of a few words. All but one, as far as I recall ("upmost" is not a word!), are the sort of errors that spell-check can't catch, which is why authors need grammar-knowledgeable beta readers. I made at least a couple dozen notes which, if/when I get ambitious enough, I'll transfer into a document and pass on to Ms. Arzú. I still enjoyed the well-told story, so it is worth the bother. I wonder to what degree the idiomatic errors stem from the author being from Guatemala.