There are lots of situations where underwater races will want to send messages to each other, write in spell books, create maps, etc.
Are there any examples out there in D&D lore (any edition, any canonical setting, Forgotten Realms preferred) of aquatic writing?
Best Answer
Eberron's Aquatic Species
The sahuagin, merfolk, and sea elves of Eberron all have sophisticated cultures. The descriptions often imply the existence of writing, but it is not often explicitly stated nor desribed.
Engraving
Sahuagin and sea elves in Eberron use engravings. Descriptions in Ghosts of Saltmarsh describe some in Sahuagin structures:
page 127:
page 130:
Exploring Eberron (p.198) has references to engravings used by sea elves:
Existence of Aquatic Archives
In Exploring Eberron, there is description of the inhabitants of the Thunder Sea. A tangential reference to the existence of written records is the undersea city Hal'daan which includes in it's description:
Magewrights and Korlass
Likely candidates, but not explicitly referenced as such, for creating written or symbolic representations of messages are fabricate and korlass. As detailed in Exploring Eberron on page 194: