I stayed at a beautiful resort in Cancun last year, and everything was perfect except for the huge amounts of seaweed in the ocean. The resort tried to remove it daily but just couldn’t keep up. No one could give me a good reason why, but another couple mentioned that the problem is widespread throughout the Caribbean. Others posited theories such as rising sea temperatures, the BP oil spill, etc. Is there a factual answer?
Anecdote …
I live at the beach near Miami. I go down to the surf most days. Some days our water has lots of seaweed. Other days it has none. Sometimes we’ll have a big influx that lasts for a week or more. Other times there’ll be just a day of heavy weed coming on shore and then none for a couple weeks.
One of the big predictors for heavy weed is high winds blowing in off the ocean for a few days. About day 3 the weed shows up en masse. Presumably the rougher water conditions are breaking it off from wherever it grows. Then it simply drifts with the wind until it runs into land.
All this varies by time of year too; there’s definitely a weed season where we are.
I’ve only been here for a few years now but there’s no long term trend I’ve noticed.
I hope we can get an expert to show up to give us the rest of the story.
I live in the area and it has been a growing mystery over the past several years. Beaches that used to be as close to pristine as can be for a tourist area are now regularly covered with 30 feet of sargassum from the shoreline, 3-6 inches deep. They go out every day carting wheelbarrows full to the jungle to try to keep the beaches clean and touristy.
This activity, more so than the sargassum itself, is causing havoc for the sea turtles that nest on those beaches and it’s an issue being heavily investigated by scientists here and worldwide.
As far as I know the mystery hasn’t been solved but a prevailing theory seems to be that it is originating in an area of the southern portion of the Atlantic known as the North Equatorial Recirculation Region and eventually washing ashore here. (a search for that phrase by itself will yield dozens of studies about the mystery seaweed of the Caribbean)
Not for nothing is that region called the “Sargasso Sea”!
At least back in the good old days. 
This. It was a weedy sea during the age of exploration.
Looks like the phenomenon has been noted and even seems to have a name. Google “sargassum crisis”.
The Wikipedia entry about the sargassum seaweed even has a subsection titled “*Sargassum *crisis in the Caribbean Sea” which reads:
Apparently, no smoking gun but a lot of suspected factors.
While this instance might be connected to climate change, I suspect that adequate historical review will show that there have been periodic blooms of sargassum. I recall stories from varying eras about weed so thick ships couldn’t get into port and so forth. (I seem to recall opposite stories of people lamenting that the seaweed appeared to have all died off, as well.)
Don’t you hate it when an article suggests that a mystery has been solved? The Seaweed Piling Up on Caribbean Beaches Is Here to Stay - The Atlantic
But then you read it and it’s not? That said, it’s got some good information.