According to ESRB, this game contains: Cartoon Violence
Parents have absolutely nothing to worry about. There is no questionable language and no sexual situations (Fish sex? Would anyone even know it if they saw it?), so it's about as squeaky-clean as they come. The graphics are bright and colorful, and the only violence is Finny eating small animals, or fighting off larger fish. The difficulty will most likely be too much for younger children, but there's nothing harmful or offensive here at all.
Action gamers may want to check it out if they can appreciate the off-the-wall adventures. It's a very short game (six hours or less) and easy for the most part with the exception of two or three segments, but it's very different and engaging. Liking either fish or weird Japanese games (or both) will probably increase your appreciation for its brand of charm.
Deaf and Hard of Hearing gamers will do fairly well, though there are a few holes in the accessibility. Most of the in-game speech is accompanied by text, but some audio speech during cutscenes is not, and the opening cinema lacks text as well. Also, there are important audio cues, like the sound of nearby predators or the call of friends in need, that are not represented onscreen visually. I would still recommend it to gamers with hearing issues, but they should be aware of these oversights.