Solid rather than outstanding, there are a couple of flaws here that make this less than the impeccable level we expect from Miyazaki. Firstly, it's too obviously influenced by The Little Mermaid - and, to a lesser degree, Finding Nemo - in its tale of a sea-creature who wants to become human, and escapes the ocean to take up residence on land. Given Miyazaki's impeccable track-record of imagination, it seems disappointingly restrictive. Secondly, fish with human faces... Sorry, it never looks other than creepy, as in Python's Meaning of Life; Pixar got it right, showing that you can have personality and character, without the need for specifically human features. Here, to be honest, I was thinking "thalidomide" any time the heroine was in fish form. On the other hand, it's still a Miyazaki movie, with everything that implies: amazing visually, and a warmth of human spirit, resulting in good triumphing over... Well, there's hardly any "evil" here to speak of, just a somewhat misguided undersea wizard (Tokoro) who resents humanity's treatment of the sea. It's his daughter, Brunhilde (Nara), who absconds and after tasting human blood (!), becomes a little girl who meets a five-year old boy Sosuke (Doi) and his mother (Yamaguchi), only for father to come after his offspring.
There are a lot of similar themes here to previous Miyazaki films, e.g. the effectively single-parent family, with Sosuke's dad a sailor who is away much of the time, or the environmentalist message of Nausicaa. But the film is never too heavy-handed in its morality, and there are plenty of warm, snuggly moments that will just make you want to hug whoever's next to you. Which could be an issue if this was a solo trip to the cinema, I imagine. However, it's difficult to get emotionally engaged in a film whose two main protagonists are pre-schoolers, even if Sosuke is highly-independent for his age (a good thing too, since his mother would be getting a call from Child Protective Services if she was in the West, between the reckless driving and the abandonments). It definitely seems aimed at a younger age-group than some of his other work, and that makes it harder for an adult audience to latch on to. It's still likely better than the vast bulk of animated features out there - certainly, kicking the arse of non-Pixar Disney - but Miyazaki gets himself held to a higher standard, and by that, this is slightly disappointing.