Dugongs and manatees
Christopher Columbus, while sailing near Haiti, believed he was looking at a mermaid when he saw his first manatee. He got close enough to note that they were "not as pretty as they are depicted, for somehow in the face they look like men." Closely related to the portly manatee, which prefers estuarine or fresh waters, the dugong is slimmer and more strictly a marine mammal, and is the likely suspect for most mermaid origin stories. To be fair to the manatee, not everyone has been quite so dismissive as Columbus. Explorer John Smith (who famously caused Pocahontas to swoon) also reported seeing a mermaid once, noting that it was "by no means unattractive."