Mermaid? Sea Witch!

'''As the ocean once again expands its domain, The era of humanity ends, and the age of mermaids begins. They occupy the newly reshaped continents and shallow seas, yet dare not venture into the deeper, more distant oceans—there lies the territory of the sirens... In Greek mythology, mermaids are known as Sirens, a type of sea demon not classified as divine. Moreover, they are exclusively female; male mermaids are the sons of the sea god. Hence, this story features only female mermaids. As for the protagonist? He isn't a mermaid~~ It's the Year of the Snake—what a shame not to write about snakes ╮(╯▽╰)╭. So, this is the story of a handsome serpent and a merman—ask me what a merman is? A...