A Lampwyrm. I’ve never seen one so close, or one so big. It’s a good thing they’re not hostile.
Some call them the “lighthouses of the sea” on account of their glowing sight. A mage skilled in beastcraft might tame one with song or intrigue one with tricks and gifts enough that it will assist them in finding land. Only once has a ship ever sunk because of a Lampwyrm, and that was because the ship crashed into it while it dozed.
Sadly their curiosity led them to be hunted, not too long ago. Lots of meat and scale to sell, you see, and the glowing oil from their eyes was often used for lamp fuel. The practice was banned a few decades back, and with one as large as this watching ships without concern, I think the ban must be working.