Oceans

The vast majority of the world is covered in water. With the continents and islands rising up from underneath the waves. There are deep trenches, rolling waves and all of it is in a vast expanse of blue waters. There are tremendous creatures that roam through the waters and shoals of fish. There are deep waters and crushing pressure. Sitting suspended within the oceans are hovering islands like what is drifting in the skies. These underwater skylands serve as peaceful locations above the deep water and homes for numerous fish and other creatures.

Fauna & Flora

The merfolk are the dominant peoples of the oceans. They’ve built castles out of coral and the fallen timber of sunken sailing ships. The merfolk have even domesticated some underwater species, creatures like seals and dolphins are treated like dogs or beasts of burden. The most powerful of the merfolk societies have even managed to tame dragon turtles. They tame these creatures as mobile platforms, seating citadels on their shells. Gigantic sea turtles and sea horses also serve as mounts and beasts of burden. Merfolk are also talented fishermen and take a great deal of effort to maintain husbandry of herds of fish. The large fish the merfolk hunt in the open ocean are traded with sailors or collected for their own feasts.
Merfolk serve as traders and also maintain a strong position with other civilizations by serving as guards and escorts for trading ships. Merfolk erectus, the merfolk of the shallows with legs instead of tails, stand on the edges of both underwater and land-based societies. They trade the bounties of the sea and the recovered treasures collected by the other merfolk for processed and forged goods from the other civilizations.
The deep waters are dark and dangerous. There are terrifying things living deep enough that not even light can reach. The powerful krakens slumber deep below, there are aboleths that muse on their ancient memories and ruminate on schemes to seize power and control. Occasionally the krakens rise up through the oceans to spread destruction or the aboleths swim into underground tunnels to begin to influence new peoples and spread their control.