There is no direct connection between sargassum drifts and red tide.  About the only thing the two phenomena have in common is that they both involve algae.  Sargassum is a macroscopic (visible to the unaided eye) brown alga, or "seaweed".  The organisms responsible for red tide are microscopic, single-celled algae, most often of the genus Gymnodinium, though other related genera are sometimes involved.  The individual organisms measure less than 0.04 mm, and are normally present in sea water in small numbers, doing no harm.  But occasionally, for reasons not fully understood, they multiply rapidly, to the point where thousands of them may be present per milliliter of sea water.  Since the organisms themselves contain a reddish pigment, the water, when so many organisms are present, actually looks red.  This is what is called a "red tide".  In addition to the red pigment, these organisms contain a small amount of a protein that is violently toxic to humans.  Bivalve mollusks, because of the way they feed (filtering particulate matter from the water), concentrate this toxin in their tissues, and become dangerously (sometimes lethally) poisonous to anyone who eats them.  The harvesting of bivalves is prohibited during a red tide, and for a period thereafter, which of course is detrimental to the shellfish industry.  Fortunately, red tides are usually quite localized.  When people hear that *shellfish* are dangerous to eat, they stop buying not just bivalves, but also snails, squid, lobster, crabs, shrimp, and even ordinary fish, though these are actually safe to eat; so these industries suffer as well.  
 
Paul Monfils