You must NOT like comedy like at all... I mean, Grand blue is damn HILARIOUS, I do agree that like almost all comedy anime there isn't a plot that forces you to watch the next episode or glues you to the screen. It was at the top of my list of anime to watch this season in a tie with Asobi asobase and some people ranked it even higher than Asobi asobase due to superior drawing and animations.

I agree on being hard to expand the plot when there isn't anything major or that makes up twists, but then again, what comedy anime does that? They're always built around the experience of someone or a group of persons.

I'm going to come clean and say I didn't like the review, not because it's poorly written, but because you're projecting the idea that it's a show just on average and a filler when it's actually one of the best that aired this time, it's just that it's not a battle manga or any of the other archetypes.

As for the diving part, you took a wrong turn, it's NOT about diving, it's about SUMMER and the experiences associated to it typically speaking. In fact in the opening song they say at one part: "Leave it to the summer" as if solidifying the idea that this isn't a sports anime but instead one about relaxing in summer with friends. Try to judge it like that instead of focusing on the diving, because the show itself doesn't focus much on the diving either. For the classical sports anime Hanebado! does a much better job.