If you are referring to the Red Tide, then, it is a form of algae called "Gymnodinium breve". Not everyone agrees on the reason the algae booms, and causes the water to appear red, but it is said to be a natural phenomena and it's not good for the ocean life or people. you shouldn't eat fin fish that are from a red tide area, red tide produces potent neurotoxins.
George A. Rounsefell has written: 'Large-scale experimental test of copper sulfate as a control for the Florida red tide' -- subject(s): Copper sulphate, Gymnodinium breve 'Contribution to the biology of the Pacific herring, Clupea pallasii' -- subject(s): Pacific herring, Pacific herring fisheries 'Factors causing decline in sockeye salmon of Karluk River, Alaska' -- subject(s): Salmon fisheries 'A method of estimating abundance of groundfish on Georges Bank' -- subject(s): Fish populations, Groundfish fisheries, Estimates, Methodology