Sea Serpent

Sea Serpents were massive sea-dwelling creatures that lived in the seas in Narnia. Sea Serpents were generally hostile to seafarers, but also rather stupid. Presumably, Sea Serpents resided in the Silver Sea, near Aslan's Country. It was also one of three sea beasts that 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 against it, 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, a sea serpent is shown to be Edmund's worst fear, and is thus encountered at the 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 boat. Eustace (in dragon form) engages the serpent, attempting to maul it. The sea creature clamps its jaws shut on Eustace, and drags him beneath the water. It then slams him into a rock formation, but the beast is fended off with a blast of fire to its mouth. The serpent turns its attention back to the boat. At Edmund's advice, Caspian steers the ship toward the rocks. The sea serpent is rammed onto the rocks, but survives, splitting 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, and the creature is finally destroyed when Eustace reunites the swords.

This causes Edmund's sword (Rhindon) to glow, and, as the serpent lunges at him, he rams the sword into the roof of its mouth. Instantly, bolts of energy illuminate the beast, and it sinks beneath the waves.

Trivia

 * The death custence of the sea serpent bears striking similarities with the death of 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 mouth from between their jaws, the major difference was the Sea Serpent's death custence contains less gore and no blood at all.