I wouldn't be too concerned about the jellyfish. If it's a REALLY big invasion and the beach is closed, sure, stay out of the water.
That being said, I can't even count how many times I've swam/snorkeled when jellyfish or Portugese man-o-war were present and I've only been stung once waaaay back in 1987. The day when 900+ people were treated was an anomaly. An influx is NOT like that scene in Finding Nemo when Dori and Marlin go swimming through the jellyfish.
On a side note, if you see Portugese man o war on the beach, make sure you keep your slippers/shoes on. If they get covered with sand, you won't know you stepped on one until it's too late.