And you can tell why they had hope for this movie: it's technically just a series of gags and action scenes, but it feels like they really tried to make it the most exciting series of gags and action scenes out there. Everything from the fun direction, wonderful animation and musical accompaniment to the stylish background art reeks of talent an enthusiasm. The animation ranges from traditionally fluid to more cartoony and loose depending on the animator and the tone of the scene, but it's generally very well made. The characters are lively, cartoony and expressive, moreso than... pretty much 99% of all anime ever. The action is smooth and fluid, the scene composition is great and Miyazaki and Yoichi Kotabe's scenes even show some really impressive water animation. While there are some cheaper-looking scenes, it never looks like it came from the era where shows like Zambot 3 (where individual episodes were rumored to have been animated *in a matter of hours by a single animator*, only some of which were actually particularly skilled... or even knew what the hell they were doing) were deeemed perfectly acceptable. But more on that later.
For a movie containing on-screen deaths and a villain that attempts to murder the child protagonists more than once, Animal Treasure Island is a pretty joyful and light-hearted ride. It's the kind of movie where even when something really tense is going on, you know everything will turn out okay anyway and the good guys will prevail, either through their own wits or through ridiculous coincidences... but that's honestly okay in this type of movie, and to fault it for this would mean criticizing it for what it wasn't trying to be. It's all about careless fun, and the characters mirror this pretty well; despite being a little on the dense side the male lead is very fun to watch, and despite being a total scumbag the pirate captain becomes oddly likable too. Kathy, the female lead, was either Miyazaki's own idea, or simply an idea that he quickly grew to like - she's a tough, surprisingly badass character that always manages to one-up the bad guys in some way... even at the point where it seems like she finally became a stereotypical damsel in distress.
In short, this is a worthwhile movie as long as you don't expect to take much away from it beyond childish fun. Hell, if this video of Miyazaki's scene from the movie doesn't make you want to give it a shot, nothing will: