Sargassum is that brown seaweed that you see often on the Gulf Coast in summer and fall. It is brown, thick and never is/was attached to the ocean floor. Off the east Coast of the USA is a "sea" surrounded by four major ocean currents. It is often calm and full of sargassum.
Sargassum is a great habitat for small fish and other ocean creatures because the thick mats give good cover. There is one small fish whose coloration and fins are such to make it blend in beautifully with the thick sargassum-google it.
I understand that it is being used as a fertilizer some places. That would be neat, for there is certainly plenty of it.
This wasn't the worst of it. I don't like to take my phone to the sandy/salty beach, but I did make an exception for this.
G^d's wonderful world--including seaweed.