I would argue it is yuri, but the ending feels a bit abrupt, not definitive but there's a strong relationship for sure. I think people didn't like the ambiguity but I thought it was fine and very sweet. It's very good either way though, strong Ghibli vibes and a very interesting take on The Little Mermaid. I liked it a lot.

I've yet to watch it myself. I posted here because the yuri tag is present on the danbooru entry for this.