Other forms: sargassos
Use the noun sargasso when you're talking about a particular kind of seaweed that's mostly found in the Atlantic Ocean, including the region known as the Sargasso Sea.
Most folks are more familiar with the place than the name for it; sargasso is a kind of brown seaweed that floats in huge masses, sometimes so dense that you'd think you could walk on it (though this is not recommended). The alternative name, gulfweed, is a nod towards the Gulf of Mexico, home to quite a lot of sargasso.