The location I may have put the most work into in my first campaign was an enormous hidden library and I loved it.

I was particularly inspired by the artwork of Giovanni Battista Piranesi and funnily enough years later I read the amazing novel Piranesi which takes place in an infinite, beautiful space with flooded districts.

I think my experience sounds less helpful, your idea sounds a little more mystical like The Library from Avatar: The last Airbender while mine was closer to that space at the end of Raiders of the Lost Ark where the government keeps all their anomalies.

But regardless, this is a library, so for the few who can access it, it's still used for study. Information may not just be in books, but there may be marine alchemy stations, underwater instruments and weapons kept like a museum. Another option is times that let you learn how to overcome the resistances or immunities of underwater monsters, or deal extra damage to them, or even control them.

Also if people often get lost in the library, what happens to them? Are there corpses in the library or does someone clean them up? What happens to their items?

Finally I'm gonna pitch something I heard way back on a blog that I thought was really cool, and I'm assuming you're running 5e, apologies if I'm way off the mark. The ethereal plane has no water, so it's "oceans" are filled with ladders, ropes and bridges. Throughout this section of the library, you can lay down this lore, and eventually reveal a spell that lets you ignore water while submerged in favour of this mythic space of snakes and ladders. If some enemies or even a couple of PCs get their hands on this spell (or magic boots or whatever), underwater combat suddenly becomes very dynamic, so some creatures swim and others run and climb in the same space.