OK, why would they organize a race in which all of their swimmers are at a high risk for jellyfish stings? Isn't the Great USofA the land of lawsuits and high insurance premiums...races don't have the money for that kind of thing...
It must NOT be as serious as everyone thinks.
Anyway, the best case scenario is nothing happens! and the worst is to say "I got stung by jellyfish" and be irritated for some days. I got stung quite a few times as a child, sat on the beach and cried for a half hour and then went back in and swam around the rest of the day...(of course had to be all calamine lotioned up at night). (and I am sensitive to everything, get hives when I get bit by a mosquito)