I think the idea that the book is a "setting book" at all unlikely. I don't think we will be seeing another setting book for at least a year after Eberron, which is itself still two months out.I take it those are your arguments for Zendikar being the next setting? I mean Zendikar does have Kraken, I remember that much.
These classes are being floated for a splat book. It's been a while since Xanathar's, and we know it sold very well, it would be foolish not to have another one in the pipeline. The book may have some kind of linking theme, but it will be a splat book not a setting book.
As for this particular subclass, I doubt it will even make the cut, being both mechanically boring and thematically unnecessary (GOO warlock already covering krakens).