I learned about the Caribbean's Sargassum problem when I was planning the DR trip. Sargassum is a brown seaweed with little berry like pods that help it to float.
I've seen small amounts of it wash up on beaches in India, but Caribbean beaches see Sargassum at a much larger scale, and this problem has been getting worse over the past decade.
This is what a beach looks like when Sargassum gets really bad:
It's possible the increase in seaweed is due to some combination of 1/ warmer oceans due to climate change, and 2/ fertilizers washing off into the ocean.
The takeaway for tourists is to either come at a time when Sargassum isn't a problem (I've heard Nov - Apr is ok in the DR), or to find a beach that is unaffected due to other reasons. In the DR for example, most Sargassum comes from the Atlantic ocean, so a beach facing the Caribbean sea will typically be fine.
I stayed in Bayahibe, in the South side of the island for this reason, and haven't seen any seaweed here at all, but there have been reports that many beaches on the east side (Punta Cana) are basically unusable at the moment.