What is the "creep from the deep"? Chapter 1-30

I just started reading Goosebumps's Creep from the Deep and I am wondering what the creep will be. A monster? What I've learned so far is about a 12-year old boy with gills...so is he the monster?

More to come...

After Ch. 1, the boy who I thought was the monster actually turned out to be a monster fighter in the sea. The boy's name is Billy and in this chapter, he goes underwater to fight the worst monster of them all: The Albino Electric Eel. He put the eel in distress and received 2000 volts of electricity, but it didn't even phase him. He has super powers, I think. While he was fighting the eel, he heard someone calling for help, so he stopped fighting the eel and went to help the person.

After Ch. 2 this is what happened.  At the beginning of this chapter Billy bobs up to the surface of the water. Then I  found a new character. He was a scientist and Billy's uncle named Dr.D. At the endof the chapter Billy learns a little something about what is called a fishing spear.

After Ch. 3, I also found a new character. It was Billy's little sister Sheena. Sheena is a ten year old. More to come...