The Dark Tide is a book with excellent characters, amazing storytelling, and a plot that'll make you never want to put it down. In a setting where mundane people and magical events collide, once you start reading you won't be able to stop.
While in the moment, the impending threat of the tide and the obvious mischief Marcin was up to kept me from wanting to stop reading in the moment, the worldbuilding behind the book was the part that kept me coming back when I put it down. Having a self-consistent world with its own mysteries, secrets, and history isn't something I see done well often in other fantasy books, and even then its normally in series and spread across books. It felt like everything that happened was a result of the characters actions and the results weren't just a case of what was most convenient for the plot.
The ending did leave me on a bit of a cliffhanger with unanswered questions, such as why the tide needed the initial sacrifice and then kept coming back for more. While it does leave room for some expansion, such as in an epilogue or a sequel, it doesn't leave a defined antagonist left besides the tide and the mainland, and I can't see the tide serving as an active antagonist instead of a passive one. However, I'm not sure it needs to be changed much, if at all - the way it ended the book was a great way to tie together a fair amount of lose ends while closing the book on a happy note.