Let's start with the fact that Percy is 12. Now I understand the need to make characters older in movies, but the inconsistencies don't stop there. Next, they took out the boarding school aspect. Not an important point, but it demonstrates his longing to be closer to his mother. Then they took out the fact that Camp Half-Blood was run by Dionysus, God of Wine. The inconsistencies get bigger from here.
From day one, everyone knows who Percy's father is including Percy. One of the biggest themes in the book is his abandonment he feels towards his father and his need to be loved by the one person he felt never loved him. It's the realization that Poseidon is his father that begins to stir bigger trouble for Percy and he feels its just one of many ways his father has let him down.
My biggest disappointment with the movie is the quest. Percy is asked to find Zeus' lightening bolt and stop the battle of the Gods from happening. That was the quest he was given. His personal goal to save his mother is what arises from that. No where in the book does it have him searching for pearls. He's given one along the way by an ally and that's it. He also discovers who is truly behind stealing the lightening bolt as well as Hades' Helm of Darkness. And, yes, while Luke was the thief, the idea was not his. It was all perpetrated by Ares, God of War and I don't think I like the fact that they made Luke responsible for all of it. Do you?