It's hard to dispute a sea serpent sighting when the captain of a naval ship and several members of his crew all claim to have witnessed it. That's just what happened in August 1848, when the HMS Daedalus, a member of the Royal Navy's fleet, was sailing to St. Helena in the South Atlantic. The ship's captain, Captain McQuhae, as
well as several members of his crew, claimed a sea beast approximately 60 feet in length passed by their vessel with 4 feet of its head raised out of the water. It passed so close to the ship that the captain suggested that had it been an acquaintance of his, he would have recognized him.
HMS Daedaluss

It remained in sight of the crew for 20 minutes and one member of the HMS Daedalus noted it looked more reptilian than serpentine. The sighting became rather famous and remains the only instance of an encounter with this particular water monster. It was reported in the Times of London and today remains a mystery.