Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:16): The saw-fish [serra] is so named because it has a serrated [serratus] crest, and cuts through boats when it swims under them. - [Barney, Lewis, et. al. translation]

Vincent de Beauvais (Speculum Natural, Book 17, 127): The saw fish swimming hidden beneath the ship cuts through its bottom, so that as the water rushes in, it drowns the crew by its crafty device and gorges itself on their flesh. (Druce translation)

Philippe de Thaon [12th century CE] (Bestiaire): "Quaut veit nes en mer halt . si se leve en halt. | A la nef fait grant laid . ke devant le nef vait | E si retent le vent . que ele, nen ad nent. | Ne la nef entant deure de nent ne pot cure." (When it sees a ship on the high sea it rises up. | To the ship it does great harm, for it goes before the ship I And holds off the wind so that it gets none of it. | Nor can the ship all that time sail on at all. - Druce translation)

Guillaume le Clerc [13th century CE] (Bestiaire): "Les mariners qui par mer vount. | Ne la querent ja encontrer. | Quer cest un grant peril de mer. | Si fait sovent la nef perir. | A qy ele put avenir." - British Library, Cotton Vespasian A. vii. (The mariners who cross the sea | Are not wishful to meet it: | For it is a great peril of the sea. | It often makes the ship to founder | To which it is able to get near." - Druce translation)

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.43, 6.44): [Marine monsters 6.43] Serra is a marine beast, as Isidore says, with a huge body and very broad and enormous wings. When this beast sees a ship sailing in the ocean, it raises its wings above the water and aims to sail in competition with the ship. And when it has sometimes attempted this for thirty or even forty furlongs and has not won, at last, not being able to endure the effort, it fails, and laying down its wings, draws itself back. The waves, however, carry the tired serra back to its original place in the depths of the sea. [Marine monsters 6.44] Serra also is another monster, as Isidore and Pliny say. It gets its name from the fact that it has a serrated crest. This beast, secretly swimming under ships, cuts them at the bottom so that the water comes in, and so drowns men by its crafty cunning and is satisfied with their corpses. - [Badke translation/paraphrase]

From British Library, Sloane MS 3544 [late 13th century CE]: "There is a beast in the sea which is called a saw-fish, and has immense wings. When this beast has seen a ship making sail on the ocean, it raises its wings above the water and competes with the ship in sailing. (But when it has competed in sailing or racing against the ship) for 30 or 40 furlongs, being unable to sustain the exertion, it gives up, and lowering its wings draws them in. And the waves of the sea carry it back again, tired out, to its own place in the deep. - [Druce translation]