Lisa Brinkman 
Myth, Fantasy, Legend or Truth [EPUB ebook] 

When a group of teenagers go on a final all day and all night outing into the ocean and carelessly throw their garbage into the water, a giant wave comes and sweeps one of them overboard. He finds himself greeted by a mermaid, who takes him to the undersea city to learn about water pollution and how to teach others about keeping the ocean clean.