Out of the Sea

Merman prince Hiccup Haddock yearns to learn about the human world, and when he saves the Viking Astrid Hofferson from drowning he decides he'll stop at nothing to win her heart...even if it means selling his voice to the Sea Witch Excellinor, who is plotting to overthrow his father and take over the sea. Basically Disney's "The Little Mermaid" with the HTTYD characters.