These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Not only is Marco's motivation and backstory drastically different, but he has the personality of a Boisterous Bruiser.
Likewise, Peter's religious beliefs were never mentioned in the manga, while the movie makes him a devout Christian who's always wearing a cross pendant.
And then there's Tim, a fearful but plucky kid in the manga who is rightfully terrified of the monsters trying to kill him, but in the movie he is barely fazed by all the violence and compares the whole thing to a video game.
Inferred Holocaust: The state of the world outside the island is left deliberately vague, though there is at least some hope: Marco points out that other people infected with Medusa would be able to create their own monsters to fight Zeus's.