You're right but it was not taken specifically for the contest. I took this photo last summer when I knew nothing about this contest.
It is one of the few I have with any warning signs on it.
It shows the general concept of people obeying the sign and swimming inside the net. That white plastic line you see in the water is the top of the stinger nets which are supposed to keep out the stingers.
At the time, I wanted to get the rescue raft and flag into the photo as well. It all helps to tell the story.