I am building an utterly MASSIVE dungeon that is based on the idea of a hidden library. Its ancient, full of lost secrets and treasure. It shifts and moves and its easy to get lost.
I'm trying to populate sections with relevant loot for my party. I have RNG tables, but I prefer not to use them, as I like having narrative reasons for why something is there. (For instance, you find a dead adventurer, and in their pack is healing potions and a magic item).
There is a section of the library that is completely underwater. Its for housing the literature of underwater civilizations - "books" that would dry out and be lost if they ever left the water.
What sort of magic items would be found in this section? Bonus points if its book-themed. (I've already got the obvious ones that give you water breathing or swim speed, help me be more creative)
If you're communicating the fact that the books and things can't be removed from the water without damaging them to the players, you could play into that with the loot. Things like powerful scrolls or even entire spellbooks, for example - but if they're removed from the water, they quickly become useless. Let your party's wizard figure out how they're going to copy that spellbook when they have to do it underwater. Maybe they'll keep the scrolls in bottles filled with water until they need them.
You could also include traps specific to that area. Think like, for example, a magical barrier that traps anyone who steals the books in that section - and underwater - until they put the things they took back. Maybe it malfunctions, and doesn't de-activate.
It's hard to give specifics, because you didn't mention what level the party is expected to be when they get to this section.