‘Exploring Eberron’ by Keith Baker, available on DMsGuild, has a nice section on underwater civilizations, quite flavorful and compelling.
Chapter 4: Thunder Sea
Exploring Eberron
Candlekeep Mysteries: Introduction
“Meet the anthology of adventure authors who have gathered to tell the latest fifth edition tales—many of them new alongside the existing team.”
Van Richten’s Guide to Ravenloft
“Design Lead F. Wesley Schneider describes his colleagues as ‘a team full of horror...