Rolling in the Deep by Mira Grant

When the Imagine Network commissioned a documentary on mermaids, to be filmed from the cruise ship Atargatis, they expected what they had always received before: an assortment of eyewitness reports that proved nothing, some footage that proved even less, and the kind of ratings that only came from peddling imaginary creatures to the masses.

They didn't expect actual mermaids. They certainly didn't expect those mermaids to have teeth.

This is the story of the Atargatis, lost at sea with all hands. Some have called it a hoax; others have called it a maritime tragedy. Whatever the truth may be, it will only be found below the bathypelagic zone in the Mariana Trench…and the depths are very good at keeping secrets.