I thought the plot was all right. However, it doesn't really change the fact that I... you know... hate/dislike May.
Speaking of May, one thing I can comment was something I hated about the movie (other than, you know, her being the focus) was the fact that May had acted like a coward near the end of the film. I mean, she raised Manaphy like she would her child, and want to know what happened immediately after Samiya rose above the waves after several centuries? Phantom kidnapped it, she pretty much let him get away with it. You'd think she'd try and save Manaphy, due to it being her imprinted child and all that, if not by swimming after him, then definitely by sending out her Squirtle after it (Squirtle is world-renowned as swimming faster than a Jet Ski.), but nooo.... She just had to stand there in fear, not even try to save it, letting Phantom get away!!!
I thought Manaphy was an all right character, and the concept of a water pokemon haven is quite interesting. In fact, this movie was what partially gave birth for my idea of Super Mermaids/Super Misties.