Miyazaki's latest. Watched it tonight (in my preferred format of subtitled, from an illegal theater copy obtained some time ago.) I had heard it was based off a children's book, but the Wikipedia page makes no mention of it and attributes it all to Miyazaki.
It's a charming retake on The Little Mermaid, with five year olds instead of teenagers, and matchmaking parents. The undersea king was once a human himself, and somehow fell in love with (and had kids with) what appears to be Owati-tsumi (the Japanese sea goddess).
US version comes out next week (Friday the 14th.) Take your kids to see it. It's going to be a classic like all his films, but I suspect teenagers might not be as appreciative as other age ranges (because the main characters are five.)