You can't see it, or touch it, but you can feel it
"I was coughing a lot, and I started to get a runny nose."
Leah Sofer of Connecticut is battling symptoms associated with Red Tide.
It produces a toxin which makes breathing tough and kills fish, though there weren't any dead fish on Marco Island's South Beach Monday.
"We arrived probably around 1:30 today, we've been out there for a few hours now, but we thought it was just getting too much and everyone around us was coughing," said Kathy Kahlhammer of Chicago.
And they weren't alone, others like Richard and Claire Moses of New Hampshire, also cut their beach day short.
"Ever since we got here we feel like we had something in our throat," said Claire.
Swimming is safe when Red Tide is present according to state officials, although some people may suffer skin irritation.