It Came from the Deep - A novel by A G Cascone(The seventh book in the Deadtime Stories series)
(1997)
Randi and Garrett are in for the vacation of their lives in the coolest resort in the Caribbean. There's sun, and sand, and water sports galore. Unfortunately, something terrible is about to happen. Because there's a struggle going on in this tropical paradise. A war of the worlds between people who live in a hidden city at the bottom of the sea and the web-footed, gill-headed mutants who secretly run the resort.