The Aquatic Races in the Sea of Wrecks

 (I'll do in depth versions of these sometime in the future, but this is just for the Sea of Wrecks)

Koa-Toa

"Yummy Yummy Yummy Chummy for my Tummy! ssssp"

Koa-Toa, are fishmen, are not insane, merely have a religion that is very alien to land dwellers.  They have a deep reverence from the sea and Koa-Toa religion is a mix of shamanism, animalism, and polytheism.  They see spirits everywhere, their gods or spirts are often other sea creatures or aspects of the ocean themselves, and they have a divine right to protect the oceans and its creatures.  To what extent they are to protect the ocean and its creatures (along with what creatures) vary greatly from Koa-Toa to Koa-Toa.  They are best thought of as man like fish.  They are semi amphibious - they can breathe air, but need to constantly be wet with salt water.

Sahuagin

Jawsome

Sahuagin, are sharkmen, and are very insane.  They are related to the Koa-Toa, (much like how a human may be related to an orc), they also worship things from the sea, but focus more on the might makes right and apex predators of the ocean.  They ritualistically butcher and then devour their captives.  They like Koa-Toa have legs with webbed feet but are much better swimmers than the Koa-Toa and are much worse on land then they are.

Fin-Folk

This is the closest I could find

Mermaids are known as Fin-Folk.  They live near the bottom of the oceans in great cities made from coral.  They rarely come to the surface, typically only to trade for metal goods.   They can survive out of the water only for moments at a time.  They do not have legs but big tails.  They are best thought of as fish like men.  They do not like most land dwellers.  The Fin-Folk in the Sea of Wrecks are on the fringe of Fin-Folk society.  They are rugged survivalists and fierce individualists.  They herd Seacows and ride Hippocampi around.  They wear cowboy hats. (They are !notTexans)