Aristotle [ca. 350 BCE] (De animalibus, Book 1, 1.6; 2, 10.1; 9, 25.4): [Book 1,1.6] Animals also differ in their manner of life, in their actions and dispositions, and in their parts. ...others are without feet, as the water-serpent... [Book 2, 10.1] There are also serpents in the sea very like those on land, except in their head, which is more like that of the conger. There are many genera of sea-serpents, and they are of all kinds of colors; they do not exist in the deepest part of the ocean. Serpents have no feet, like fishes. [Book 9, 25.4] The marine-serpent, in color and in the form of its body, resembles the conger, but it is darker, and more powerful. If it is captured and allowed to escape, it buries itself in the sand, which it pierces with its snout, for its snout is sharper than that of a serpent. - [Cresswell translation, 1887]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.23): The sea has many serpents, and none of them have feet, but they crawl like eels in the sea and only swim when necessary because they have no fins. They are like the serpents of the land except for the head, because their head is very hard and rough. They are often more malicious than the land snakes. And it should be noted that they do not live in deep water, but in the water near the shore and around the rocks of the sea. - [Badke translation/paraphrase]