This has to be one of the very worst boox I've ever read. It's one of those mysteries where noted writers write a chapter, leaving a cliffhanger for the next author to deal with, who then does the same thing to the next author. These are good writers, and I've read some of this sort of story before without being displeased. However, The Sunken Sailor is just plain awful.
Having read every word, I still have no good idea who is who and what their real role is. And the prime villain is someone who disappeared early and entirely from the book and who really had no good reason for being cast in that role. The mop up chapter was as inexplicable as the rest.