Charybdis

Charybdis was once a beautiful naiad, the daughter of Poseidon and a nymph. She takes form as a mouth that swallows huge amounts of water three times a day, creating whirlpools. Charybdis was very loyal to her father in his endless feud with Zeus; it was she who rode the hungry tides after Poseidon had stirred up a storm, and led them onto the beaches, gobbling up whole villages, submerging fields, drowning forests, claiming them for the sea. She won so much land for her father's kingdom that Zeus became enraged and changed her into a monster forever.

The myth has Charybdis lying on one side of a blue, narrow channel of water. On the other side of the strait was Scylla, another sea-monster. The two sides of the strait are within an arrow's range of each other, so close that sailors attempting to avoid Charybdis will pass too close to Scylla and vice versa. The phrase "between Scylla and Charybdis" has come to mean being in a state where one is between two dangers and moving away from one will cause you to be in danger of the other. "Between Scylla and Charybdis" is the origin of the phrase "between the rock and the whirlpool" (the rock upon which Scylla dwelt and the whirlpool of Charybdis) and may also be the genesis of the phrase "between a rock and a hard place".

The Sea of Monsters
Clarisse takes her ironclad between Charybdis and Scylla to enter the Sea of Monsters, which is actually located in the current day Bermuda Triangle. She is a black coral reef with a tree on top and described by Percy Jackson as a "dentist's nightmare", a huge disgusting mouth with braces underneath. She is surrounded by the whirlpool she creates when she sucks up the sea and everything in her range. Clarisse chooses to take on Charybdis because she was an easier target for the cannons.