Ponyo on the Cliff by the Sea (2008) Anime Watch

A young boy, Sousuke, befriends a strange looking goldfish whom he names Ponyo. Unbeknownst to Sousuke, Ponyo is a magic fish who has decided that she wants to live with Sousuke and the other humans. Unfortunately, Ponyo's decision to give up her underwater life creates a crack in an ancient magic spell, and places the world in danger. Together, Sousuke and Ponyo must set things right.