The Mermaid Of Black Conch By Monique Roffey

Near the island of Black Conch, a fisherman sings to himself while waiting for a catch. But David attracts a sea-dweller that he never expected - Aycayia, an innocent young woman cursed by jealous wives to live as a mermaid.
When American tourists capture Aycayia, David rescues her and vows to win her trust. Slowly, painfully, she transforms into a woman again. Yet as their love grows, they discover that the world around them is changing - and they cannot escape the curse for ever...