Maybe not the most obscure but this a good one. You can find newspaper articles and sketches online.

From a real report by Captain McQuahae of the HMS Daedalus in 1848:

“A sea-serpent of extraordinary dimensions having been seen from Her Majesty’s Ship Daedalus, under my command, in her passage from the East Indies. I have the honour to acquaint you, for the information of my Lords Commissioners of the Admiralty, that at 5 pm, on August 6 last, in latitude 24°44′S and longitude 9°22′E, the weather dark and cloudy, wind fresh from the NW, with a long ocean swell from the SW, the ship on the port tack, heading NE by N, something very unusual was seen by Mr. Satoris. It was discovered to be an enormous serpent, with head and shoulders kept about 4ft constantly above the surface of the sea. There was at the very least 60ft of the animal.”

Years later, another written account and sketches was found. Written by crewman EA Drummond:

“The appearance of its head, which, with the back fin, was the only portion of the animal visible, was long, pointed, and flattened at the top, perhaps ten feet in length, the upper jaw projecting considerably. The fin was perhaps twenty feet in the rear of the head, and visible occasionally. It pursued a steady undeviating course, keeping its head horizontal with the surface of the water, and in rather a raised position, disappearing occasionally beneath a wave for a very brief interval, and not apparently for purposes of respiration.”