Related to the jellyfish, sea lice are actually microscopic larvae of jellyfish and other types of stinging organisms in the ocean. There are many varieties and, frustratingly, they’re basically impossible to see with the naked eye. In Key West, they often colonize on seaweed floating in the ocean, so try to avoid this while swimming.
If you get hit by sea lice, it’s usually no more than a temporary mild itch or burning sensation that goes away as quickly as you feel it. On rare occasions, the sting of sea lice is delayed and can be rather painful and uncomfortable for days.