Stingrays often lie hidden under the sand in tropical areas and even as far north as Cape Cod and British Columbia. They have a sharp spike on their tails and will attack if stepped on. Wear sandals or booties, shuffle your feet and probe the bottom ahead of you with a stick. It's controversial in some areas where divemasters feel that gloves increase the temptation to poke and fondle delicate marine life. Sometimes headache, chills and fever.
Severity varies with the amount and kind of venom injected and the susceptibility of the victim.
The sting of a box jellyfish can kill a human in five minutes. The lion's mane jellyfish can be lethal too. These are found in temperate and tropical oceans, though the Indo-Pacific varieties seem to be most dangerous. You bumped into something. Jellyfish are just one class of the phylum Cnidaria, which includes 9,000 species of anemones, corals, hydroids and jellyfish.