If you were in the mood for an action-packed deep-sea adventure with monsters, you should check out Deeplight by Francis Hardinge. Hark, a 15-year-old orphan, has grown up on the island streets of Lady's Crave. It's a world governed by the sea and the stories of the gods that used to rule them. He's learned how to be tough, how to be an excellent liar, and to always have his oldest friend's back. When Hark discovers an ancient relic of the old gods (think gigantic sea-monsters) Jelt begins to change in disturbing ways. Hark is on a mission to keep this relic and its mysterious powers away from mad scientists, smugglers, cult leaders, and most importantly, Jelt. But can Jelt be rescued?
One of the best things about this novel was the world building. The author does a great job of transporting you to this island world and getting you to understand the types of people that live there. For example, people known as "sea-kissed" have lost their hearing from diving related incidents. The sea-kissed are revered in this society, and use unique sign languages to communicate. One of the main characters is sea-kissed, and I thought it was a very interesting way to bring some positive representation to a group of people not often featured in literature.
The other part about this book I really liked was Hark's character development. He really thinks about who he is and who he wants to be. There was this theme of finding yourself through your own actions that I really appreciated. This quote stuck with me: "Maybe sometimes there isn’t a right thing to do. Maybe there’s just lots of wrong answers, and you have to pick one you can bear—something that doesn’t break who you are".
This is a great option if you want an adventure fantasy on the scarier side (the monsters are pretty terrifying and Hark gets into some tough spots). If you liked Rick Riordan's books that weave mythology and lore into adventure and battles, this book is worth a look! There are also really cool submarines =)