They actually didn’t. This is a common example of the sloppy research many debunkers engage in (another good one is the many explanations for the “mysterious disappearances” in the Bermuda triangle… the number actually isn’t unusual at all the whole thing is a hoax the various guesses at what causes the disappearances are largely irrelevent). Manatees and the dugong (similar species) are native to tropical waters (Florida, carribean, etc). Mermaid legends are found around the world but some of the oldest are from Scandanavia. Do you know what you don’t find in the waters sailed by scandanavians? Manatees.

There is, to my knowledge, exactly one example of a mermaid sighting that probably was a manatee. On January 9, 1493 Christopher Columbus reported seeing manatees off the coast of Hispanolia (Haiti today) and described them as more masculline looking then he expected. This is the right part of the world and at least vaguely fits the topic.

As to the rest there is an explanation but its a little more complicated but it also fits the folklore vastly better. During storms there’s a sort of mirage that can form if you are standing at or below the surface of the water (the deck of viking long ships was often below sea level). This effect distorts objects and animals floating in the water and can make a seal or dolphin or even just a piece of driftwood look human. Now if you know much about mermaid legends you know that to see a mermaid or espescially to hear one sing is usually a sign a storm was coming. Aside from the musical sounds whales and dolphins make this actually fits the storm aspect of the mirage very well since on old sailing ships the wind whipping through the sails and rigging often can sound very musical. This explanation fits mermaids for better then manatees (without invoking anything supernatural) but some debunkers are happy with the simple explanation and don’t bother to do any serious research.