https://thebeachreview.org/2014/06/17/florida-beach-warning-flags/
olid Purple Flag- Dangerous Marine Life
Often times, this flag will be flown with one of the other flags. For example, you will see a green flag flying, which means water conditions are favorable, but the purple flag is also flying just beneath it, because the lifeguard has spotted man-o-war. You can still enter the water, but be aware of what is swimming around you. Be particularly wary of clumps of seaweed, where jellyfish may be. The lifeguard will usually write on the lifeguard communications board what specific dangerous marine life is in the water.
Dangerous marine life does not necessarily mean that a Great White Shark is lurking in the waters. The absence of a purple flag does not mean you are “alone” either. It is always best to be aware of what is around you. The magic of the ocean is that you may be underwater, looking out into the clear water, and there is nothing in front of you. You continue to look out longer, and suddenly things will appear before you that you wonder how you could not have seen. It’s awe-inspiring.