Kos ran to the side of the ship, the rain still pouring down in sheets. He looked for any sight of the serpent. Finally, it poked its head just above the violent waters. Seeing his chance, Kos leapt down, his sword still in his grip. He landed on the creature's head, and locked himself into position using his legs. Grasping the greatsword with both hands, the dracon plunged the blade into the serpent's head, sending it through the creature's brain and out into its mouth. The sea serpent hacked and coughed a few times, sending blood into the turbulent sea before flopping over to its side, dead. 

Kosjurec pulled his sword free from the serpent and swam to the side of the ship, where ropes had already been lowered for him to climb. Of course, he could already climb to some extent, given his claws and the rough side of the ship. The ropes were needed to help in this wet and slippery environment.