Check for an allergic reaction: typically difficulty breathing, dizziness or heart problems. Another weird sign of an allergic reaction is profuse sweating, which might be hard to spot in someone who has just emerged from the water.
If you suspect a system-wide allergic reaction - as opposed to localized discomfort caused by the sting itself - call 911. "Respiratory distress or associated cardiac problems? That's a 911 call, absolutely," Mendofik said.
Once an allergic reaction is ruled out, you can proceed to treat the sting.