I absolutely enjoyed this film. It's about a beautiful friendship between kids from different worlds. And whether this was intentional or not, it is also about how "generational hatred" towards groups are quite meaningless and how new generations can bridge age-old divide, share common ground and succeed together. The people were hell-bent on catching the sea-monsters, but not one of them were actually victims of the monsters. You could also see that there were photos and sculptures of sea monsters being killed by humans, representing historical accounts. Why were they killing sea monsters? It was unclear. And when you think about world history, why did any one group kill another? From a modern-day POV, it all seems so absurd.
The villain, Ercole Visconti, represents "old institutions" that have been consumed by arrogance and narcissism, and bully those who do not fit in or refuse to "bow down" to his "superiority". The way he treated his friends or allies was abysmal--his success was hinged upon THEIR efforts, but throughout the movie, he takes credit for it as if he was the lone winner. And if they failed, he would hurt them. Can't help but think of that in the context of global politics.
At its simplest, this movie is a great learning for kids on how to collaborate in a team and overcome differences.
Pixar really does an amazing job with incorporating complex adult topics into their stories, like what they did with Up and Inside Out.