Stede Bonnet.

that day for the curious crew of The Revenge, the ship ran aground on an almost deserted island so the captain ordered...vacation for the moment until they manage to get the ship back into the water so the entire crew dispersed and went about their business, Stede was not the exception, so he decided to get away from the ship and the crew a little while walking along the shore of the beach

Mermaids were a myth, one well known among pirates but that stopped there...a simple myth, beautiful creatures with a divine voice but they were dangerous and one was stalking Stede