If there is a swarm of jellyfish, there would be signs posted. However, you should always be alert while swimming no matter where you are.
Jellyfish are all over the place in the world. You need to look around underwater while swimming & check your surroundings. It would be foolish to assume you are always safe in the water.
I was in the Bahamas several years ago swimming over by the Atlantis. Had I not looked around, I would have swam into a clear jellyfish that was about a foot wide and its tentacles easily had to be 3 feet long. It was about 15 feet away from me and I was only in about 10 feet of water.
It was the only jellyfish around & I warned several other swimmers nearby that there was a lone jellyfish in the area.