We studied this sign because we had been warned not to even put our toes into the water, outside of the netted areas with lifeguards. And then, we paid $10 extra to rent thin head-to-toes-to-fingers wetsuits when we snorkeled on the reef. Good thing, because there were lots of jellyfish out there and we didn't really know which were harmless and which were deadly. Several people went without the extra protection. Yikes.
From Australia: Cairns Beaches