The Sea Wasp (Carybdea alata), in the found in the West Indies and Caribbean, is a potentially dangerous jellyfish, especially to some individuals, although not as deadly poisonous as the Pacific Sea Wasp (Chironex fleckeri). It is reported off Cozumel, thought I've only seen it in the Bahamas.
It is a box jellyfish (Cubomedusae) with a small, four-sided, bell- shaped body, up to 2 x 3 inches, though often resembling a one inch "cube." Its four tentacles average about 12 inches long, one attached to each bottom corner of the body.