This is some work I did for the artstation 'beneath the waves'challenge I unfortunately never managed to finish due to a lack of time.
My idea was to have a coral-based (or loosely inspired by coral) folk who don't live too deep, and occasionally visit deep or shallow waters. They are a peaceful tribe that will easily accept outsiders. They are not fast so they rely mostly on farming/trading and on stealth when hunting to survive. They use sea creatures as cattle, pets or as mounts to get around faster. Some farming and building tasks are also performed by domesticaded sea creatures. Most of them are very vulnerable to the changes in the water, so the tribe needs to move from place to place to find new suitable sea beds to live and build new reefs, which is getting harder to do each time.