From what I've seen from the trailers, the script looks to be a much bigger problem than the actors here. Whenever I have seen a scene that looks off, it is not because of a subpar delivery on any of the actor's part but because the actors were told to say something weirdly out of character for the character that the actor is playing.
Prime example is that scene where Luffy says "I'm sensing some tension in the crew." with Nami and Zoro responding with "Not a crew!". The delivery was fine but the line is just not something you should see from an adaptation of One Piece. That shit just doesn't fit the characters or the style of humour in the manga/anime.