Sea Serpent

Sea Serpents were massive, sea-dwelling creatures that lived in the seas of Narnia.

Sea Serpents were generally hostile to seafarers, but also rather stupid. Presumably, they resided in the Silver Sea, near Aslan's Country. It was also one of three sea beasts that the Sea People were afraid of.

Dawn Treader Encounter
The Dawn Treader and it's crew encountered a sea serpent on their voyage, in search of the Seven Lost Lords of Narnia. It coiled itself around the ship, apparently in an attempt to break it in two. Due to the serpent's extremely strong hide, swords and arrows were useless, and only by following Reepicheep's advice to push the coiled body over the side of the ship, were they saved with the loss of only the dragon-tail stern.

Film
In the film adaption, a sea serpent is shown to be Edmund's worst fear, and is thus encountered at Dark Island.

When Lord Rhoop tells them not to think of their worst fear, Edmund unintentionally thinks of a serpent, which the Green Mist feeds off, takes the form of, and then proceeds to attack the ship. Eustace (in dragon form) engaged the serpent, attempting to maul it. The sea creature clamped its jaws on Eustace, dragging him beneath the water.

It then slammed him into a rock formation, but the beast was fended off with a blast of fire from his mouth. The serpent turned its attention back to the ship, but at Edmund's advice, Caspian steered the ship toward the rocks, ramming the sea serpent onto the rocks. The serpent survived, though, and proceeded to split its body into a cobra-like frill, covered in centipede, leg-like claws.

The crew fire multiple harpoons at the monster's underside, to restrain it, until the creature was finally destroyed after Eustace reunited the Seven Swords.

This caused Peter's sword (Rhindon), which Edmund had been using, to glow, and, as the serpent lunged at him, he rammed it into the roof of its mouth. Instantly, bolts of energy illuminated the beast, and it sunk beneath the waves, dead.

Trivia

 * The death custence of the sea serpent bears striking similarities to the death of the Basilisk from the 2002 film "Harry Potter and The Chamber of Secrets". Both were giant serpentine beings, killed by the main protagonists with a mythical sword, by stabbing the roof of their mouths from between their jaws. The major difference was that the Sea Serpent's death contains less gore, and no blood at all.