Sea Monster

From publisher blurb:

In ocean depths, murky swamps, and shaded black lagoons, where even the most daring explorers fear to swim, terrifying creatures lurk in the deep. Though called Sea Monsters, they can be found in all bodies of water deep enough or murky enough enough to hide in. When they leave their watery homes, however, they take on a shape indistinguishable from a human, allowing them to hide among the human populace.