Jellyfish

Jellyfish are free-swimming non-vertebrates found in every sea across the world.

Consisting of a gelatinous bell with trailing tentacles capable of stinging prey, they can be harmful to careless divers. While jellyfish stings are in most cases harmless to humans, some can be extremely serious, to the point of being lethal.

Reference

 * Assassin's Creed IV: Black Flag