The anaerobic breakdown of sulfates in the seaweed leads to the production of hydrogen sulfide gas.
You can always go to Fort Taylor's Beach
It's because of the seaweed. Shallow waters without much surf action will let the seaweed rot. Best beaches in the lower keys are at Bahia Honda State Park about 1/2 hour NE from Key West.
Did not smell bad when me and my wife visited.
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