Jellyfish

I'm now pretty certain that what got me the other day in the water was a jellyfish.

Jellyfish are often present in coastal waters, having been brought ashore by winds or ocean currents. They are most common in warm ocean waters, especially along the Atlantic coast of the United States. Stings result from contact with the tentacles, which trail from the jellyfish's see-through body. Jellyfish swimming in the water are often hard to see. Beached jellyfish, which may look like the cellophane wrapper from a cigarette pack, can sting if touched.

Jellyfish stings cause immediate, intense pain and burning that can last for several hours. Raised, red welts develop along the site of the sting, which may look as if you have been hit with a whip. The welts may last for 1 to 2 weeks, and itchy skin rashes may appear 1 to 4 weeks after the sting. Fortunately, most jellyfish stings are not severe. Extensive stings, allergic reactions, or severe toxic reactions are not common but do occur. To avoid the risk of drowning, swimmers should get out of the water as soon as they realize they have been stung.

src: http://www.webmd.com/a-to-z-guides/marine-stings-and-scrapes-topic-overview