It bugs me when people complain about Hollywood drivel, but then tear apart any movie that wanders too far from the plot diagram you learned in junior high language arts. Like most foreign films, Japanese dramas require a different sensibility to get the most out of them. Mizu no Onna is almost a textbook example. Sure, it has it's problems. It is a good half-hour too long, the nameless biker chick played by Hikaru is pretty much unnecessary, and I actually hit a point where I wished Ryo would put her s**t back on already. (and on a technical note, the contrast in the first third of the film was way to harsh). But the reviews people have written already make me feel like I should be defending it.
This is very much a character piece. The plot is low key, the dialogue minimal (and, as someone else mentioned, not everything is given the Dr. Watson scientific explanation, which I actually thought was a plus). But the two main characters are interesting and, with a couple of blips, well developed. UA and Tadanobu Asano do brilliant jobs of portraying two contrasting people drawn together by their respective marginalisation. This is almost entirely done through body language and the cinematography (though yes, some of the mood setting shots of rain do drag on. It's wet, we get it.) The plot isn't particularly convoluted, and quite easy to follow as long as you actually listen to the dialogue.
There are some cultural nuances that some people won't get. The bath-house culture, in particular, with all it's nostalgia and community spirit, is hard to convey unless you've been to one, and this is rather important to understanding the character development. A lot of Japanese films, even artsy ones, are quite accessible to foreign audiences, but gMizu no onnah is perhaps more 'Japanese' than most. If you can accept that, and sit through the first 20-30 minutes or so (after which it seems to find it's rhythm), it's a very attractive and thoughtful film. There are worse ways to spend a rainy afternoon.