In The Twice-Drowned Prince by L.M. Morrison, a prince is rescued by a mermaid after a shipwreck and makes a deal with a witch to become a Mer. However, the deal comes with a price: His rescuer must fall in love with him, or he will turn to sea foam. Also, the prince is still partly human and this complicates thing as the prince struggles to find balance between his two natures.
This retelling of The Little Mermaid is engaging and well-written. The author does an excellent job of addressing the themes of mercy, forgiveness, human nature and love as the prince and the mermaid learn about each other and their worlds. I like the interaction between the prince and the mermaid, and I like how the prince becomes a Mer rather than the mermaid becoming human. The role reversal of the prince and the mermaid is interesting. I would recommend The Twice-Drowned Prince.
