Dugongs are salt-water relatives of the manatees. Manatees and dugongs are commonly referred to as "sirenians" as they belong to the "Sirenia" order, the name of which is derived from the Sirens of the Greek mythology... Now, this is what they currently look like:

Between you and me, those sailors had poor eyesight and were really drunk and lonely when they mistook this 1000-pound-mammal for attractive women with fish tails! No wonder Christopher Colombus wrote in his journal: "they are not so beautiful as they are said to be, for their faces had some masculine traits." You bet!