I watched the anime, and liked it enough to read the manga as well.

It should only be natural that for a comedy type of story, the anime and manga would give off different vibes, and different, they are. They ARE, overall, pretty similar, but just the format of manga makes it slightly different, and in this case, for the better.

Honestly, Grand blue has some of cringe and ecchi moments sometimes, or just unfunny chapters. However, the pacing of the manga makes it so that you can skip it over more easily than the anime. The funny chapters of Grand Blue however, stay just as good as the anime. The exaggerated facial expressions are exactly the same as the anime, and it's honestly always damn funny whenever I see them.

And I think this is over-exaggeration sort of theme is present throughout the entire manga, and it's one of the reasons why I like it so much. Sure, on the surface level, it seems like a slice of life story, and surely, most of the chapters in the manga can theoretically happen in real life, but the charm is the embellishing it has. Sure, which college doesn't have frat parties and party university clubs? But the pab in the manga are over the top, yet not overly so, but enough to the point that it's funny. Same thing with all the character personalities. Few characters seem feasibly realistic, like maybe Chisa and Aina, but even then, their obsessions with certain things make them fit in the comedy.
All the stories involving random activities, like camping, working part time, going to classes, eating, etc are turned into something extremely funny since all the characters have an inherent randomness and potential for good interactions.
All of these small, but good things about the novel bring it up to something truly great. Even the setting of "diving club" near the sea is used in a simple, but smart way.
All of the characters (and there's a damn lot of them) are interesting and funny, and whenever they appear in a chapter, you're sure to have a laugh. It's exaggerated and unrealistic, but that's the charm of a comedy novel.