Searching for a coming-of-age sci-fi story about a deep-sea diving boy with an octopus friend.

As I said, it's a coming-of-age story that involves sci-fi elements. The main protagonist lives in a submarine station fairly deep below the surface of the sea, he uses a special, breathable liquid that allows him to dive freely at great depths because the liquid somehow helps with the pressure. I think the main plot revolves around him uncovering some huge corporate secret, important enough that they want him dead, so he can't tell anyone. He has an octopus (or a similar organism) as a friend as well as some fellow teenage divers using the same technique he uses and some adult researchers living with them on that submarine station. I'm fairly certain the book was a paperback. I sadly can't remember when exactly it is set, but there were telephones and modern diving equipment, so it must've been a set in modern times. Again, the exact length of the book is lost on me, but it was roughly 400 pages long, using other books in my collection as reference. I personally read it in German, since that's where I'm from, although I don't think that's the original language, might be wrong on that though.

The year was probably 2015/16 when I read that book, I was around 15 years old and it was age appropriate, I think. As far as I can remember, I got it from our local library. I'm fairly certain it was written for young teenagers, with it being a coming-of-age story and all. If it was new when I read it, I can, for the life of me, not remember.

Thank you in advance for helping me with this matter.