I read a sci-fi/horror book as a kid, and cannot for the life of me get the plot out of my head. Unfortunately, I also cannot recall the name or author. I've included below as much as I can recall.

The book opens on a poor shanty town near the ocean (S.F. maybe?). A young boy witnesses a monster destroy the entire shanty town in one night, killing a lot of people. The boy escapes, and grows up looking for the monster. He finds, throughout the course of the novel, that the "monster" is man that can turn into the creature at will. He finds, too, that there are others among us that can do this as well. He locates a woman whose daughter can transform in the same manner, and the daughter battles the bad guy on his boat in the harbor. Both the daughter and the bad guy go down together during the battle, presumably killing each other.

Any assistance would be much appreciated.