Question: We're coming to Florida with our kids and will be spending a lot of time swimming. Should we be concerned about jellyfish? When are jellyfish most numerous?
Jellyfish occur in all Florida marine waters. There is generally no way to predict when they will appear, although they often wash up in large numbers after several days of onshore winds. Human contact with jellyfish is mainly a problem during spring, summer, and fall, since few people venture into the Atlantic or Gulf during the coldest months of the year. I get the most reports of jellyfish sightings during the late winter an spring months--generally February through April. The heaviest concentrations of jellyfish occur on the Atlantic Ocean side of Florida and in the Panhandle region on the Gulf coast. The lower Gulf Coast peninsula--from the Tampa area south to Marco Island--seems to have fewer jellyfish.
The most dangerous jellyfish is generally the Man-of-War (which is technically not a "true" jellyfish). It is easily identified by its appearance. Another jellyfish that delivers a painful sting is the sea nettle. Less potent stinging jellies include the cannonball jelly and the moon jelly.
Some people are more sensitive to a jellyfish sting than others, but it's best to avoid all contact with jellyfish-like creatures unless you are able to positively identify them.
The best way to avoid contact with jellyfish is to look at the water's edge to see if dead jellyfish are present on the shoreline. If you see more than one or two, you should consider not going in the water, or at least keeping your eyes open. If you see Man-of-war's on the beach, don't go in the water.
There is a product that does a nice job of taking the "sting" out of a brush with a jellyfish:
Jellyfish Relief Spray by Ocean Care Solutions relieves the pain and symptoms of a jellyfish sting AFTER you've been stung. It is widely used by first responders and gets high praise.