They can pretend all they want this is some standalone water creature movie, but there's no way this isn't a Hellboy prequel and probably an origin story for Abe Sapien. I don't care what anybody says. It's Guillermo del Toro and everything points to this fish-man-thing as being Abe Sapien. Sure, they didn't title it Abe Sapien: The Movie, but it's still very clearly a movie about Abe Sapien. Even if the creature turns out to be called something else and has a completely different origin story and ends up dying, it's still an Abe Sapien origin movie. Even if it's actually a re-imagining of Creature from the Black Lagoon it's still an Abe Sapien origin movie. That's just how these things work.
UPDATE: Guillermo del Toro has blatantly denied this is an Abe Sapien prequel. But, ya know, if you ask me it's still definitely an Abe Sapien prequel.
It is not. https://t.co/smbrgqGeP4— Guillermo del Toro (@RealGDT) July 20, 2017
And here's Abe Sapien from Hellboy for reference.