Sea urchins are small, globular, spiny sea creatures that cover much of the sea floor off the coast of Dubrovnik. This isn't a bad thing being as they only gather in super clear water. I'd recommend that you wear sea socks or something to cover your feet while walking through the water. Also, if you snorkel, be carefuil you dont brush up against the ones on the sides of the rocks.
The beaches round Dubrovnik are largely rocky and therefor home to myriads of sea urchins. The urchins are small black animals living on the rocks with spikes all over them.
You can by special rubber bathing shoes that can withstand the spikes, or you can do what i did, and just keep an eye on where you put your feet.
The urchins normally do not move, and you can touch them without problems, but if you step on one it will hurt a lot, and there is a substancial risk of infection.
If you get "stung" then olive oil is recommended helping to remove the spike.
When swimming watch out for sea urchins, they could be anywhere. Please wear the shoes if not familiar with the place you are swimming at. If you get stung by jelly fish, one option is to go to the pharmacy and consult a pharmacist, if doctor is not readily available.