This was really interesting and I think the reviewers who were "lost" or "didn't get it" are probably not into death lore as much as me. I didn't read the summary or reviews before starting and sussed out immediately that they're in a purgatory and the boatman was some equivalent to Charon. While it's presented in a very Western view, Greek lore of the River Styx and Charon is the main feature.
The book is told from a small-time gangster name Gino's POV and, after waking up on the bank of a misty river with a headache and vague memories of what happened to him before, he meets a young man, Shane, who immediately vomits up blood and a coin. While Shane recovers, Gino pockets the coin without saying anything, unknowing how important it is. Since neither of them knows where they are or how they got there, they form a loose coalition and start exploring the area. However, it's quickly apparent that they're going in circles. They see the boatman who ignores them or stops to pick up other people who appeared without Gino and Shane noticing. They see some bottles that appear to hold prayers about them from people in their lives. They get frustrated and bored and spend some time in the trees together. They eventually put it together and realise what happened to them and why they're there.
Something that isn't really explained is why the boatman didn't approach them earlier, because he did approach the surfer who didn't have their fee so it wasn't that Gino stole the coin. I guess it's a higher power at work sort of thing that Gino had to learn a lesson or something. And while there are some really interesting takes on the Greek myth that I feel make the book worth a read, there isn't a very satisfying ending since there's an inevitable separation of our two MCs.

Overall, a very quick, interesting read that I would recommend if you're interested in exploring death mythos.