One of its best qualities is: much like Erika upstairs, this movie does not waste time getting to what you came to see, which, yes, is a reference only understood if you had seen this movie and remembered its short first act. It is in every way as advertised, but it does not expand far beyond that scope. The characters are very disposable and generic, the story is not very strong, and the plot doesn't do much. However, what you were promised was: an anime about walking killer fish, and no one can say this didn't provide that, so it earns its respectable B.
This is a "manga", and not being an expert on the subculture of such things, I gather that to mean more "adult" in some fashion, which I guess is to excuse the gratuitous sex and nudity. The gratuitous sex and nudity weren't doing much for me, for one, because nudity inserted into regular movies is not appealing to me personally. I didn't watch this to wank, so what is the point of a small smattering of obvious wank material? Another reason was, it is a CARTOON, and I'm not being intentionally judgy. People who get hot for sexy cartoons are already low hanging fruit for judgement and piling on while writing about a manga actually makes me the asshole. Just to say, I PERSONALLY can't get into it. Also, "manga" has no special meaning for me, so I judge what I see by normal movie standards. To be slightly forgiving, it is arguable that the sex scenes related to a minor sublot later in the movie, but technically, very technically so.
Nothing about it is life changing, but it is alright to watch for the novelty of it, and if you don't have sky high expectations. If you want a fun "popcorn movie" this is your jam. Just know there is no "message" about anything in here. The plot as it is, is just a framework to justify graphic Imagery of walking fish mutilating masses of people.