Who is the antagonist in the book the red kayak?

Well, generally, there are no antagonists in "The Cay". But, at the beginning of the book, Phillip does seem a little prejudice. He thinks he is higher value than Timothy in the beginning, but soon realizes that all people are the same underneath, just as Timothy said "I hate certain white people, but I do not hate all of them,".

All tales have an antagonist. That does not necessarily mean it is a person. In this case, the hardships and the challenges of survivability endured by the characters could be considered an antagonist.