I swear to god mangaka don't know how to write. Was he being a little too serious and trying too hard instead of just trying to have fun and make memories? Yeah. But don't just have Umi be some magical girl who saves everything and everybody. Nobody, at least I wouldn't, risk getting written up for being on school premises at night just to do some stupid riddle for literally no reason. This is why I hate shoujo manga and particularly romance manga. They shove everything on the dude and act like he did everything wrong and deserve the blame, just for the heroine be some goddess who can do no wrong.
Literally talking it out would have been more simple and just as if not even more "sweet" instead of including a shitty plotline/chapter. I also hate the fact that readers just instantly lose their brain and rational thinking as soon as something "nice and sweet" happens and just completely forget how bad an idea this was and how bad this and the last chapter were.