It is a cgi/anime movie from years ago.The world is made up of a bunch of floating islands. It starts when the main character escapes his burning island. In another scene, the boy and his friend are playing in some turrets on an airship when someone attacks. The two boys fight back and when the fighting is over, the main boy and sees that his friend was killed. There is also a giant, mechanical fish? That goes around eating and/or destroying the flying islands.

Aero-Troopers: The Nemeclous Crusade (http://www.imdb.com/title/tt0382521/reference)