From this month's Undercurrent Magazine.
"Skip The Sunscreen While Diving
It can damage and even kill coral reefs, says a study in the journal Environmental Health Perspectives. Italian marine biologists have linked four UV-blocking chemicals in sunscreens to coral bleaching because they cause viral infections in the symbiotic algae that live inside reef-building coral. The viruses replicate until their algae hosts explode, infecting neighboring coral. The researchers estimate that 5,000 tons of sunscreen wash off people in oceans annually, and that up to 10 percent of coral reefs are threatened by sunscreen-induced bleaching."
I've always advised people not to use it before diving as it just washes off (yes, even the "waterproof" stuff), but I didn't realise it was positively harmful.