I never thought I'd wish for more pollution.
Snorkeling at Tioman Island last year, I floated over a basketball-sized black sphere of horror, thick six-inch spines moving, moving in response to contact, five-toothed circular mouth lurking below. Later we bobbed above a field thickly carpeted with the poisonous nightmares, me screaming into my snorkel as we searched for a safe way out of the shallow water. Since then, not even spiders have given me the shivering terrors the way sea urchins do.
But that was a long time ago, and I didn't make the connection between "ultra-clean" seawater and the presence of urchins when we arrived at Hvar. The ramp's the soft way out of the water, and flabby from too much Italian pasta I preferred to brute-force my way on to the jetty, scrambling up the sharp rocks. So when I felt a sharp pain in my foot, I wasn't totally alarmed. Out of the water, though, I found a dozen black spines embedded to varying depths in my toe. The staff handed me a sterile needle and invited me to dig out the pieces. This is harder than it looks.
The next day I wore my flip flops into the water.