I was in Mexico last week. Stayed half the time in Isla Mujeres where there was zero seaweed. Stayed the rest of the time in Tulum where the shoreline was filled with nasty brown sargassum. Hotels try to keep up with cleaning it but it is almost impossible. The locals were saying it has been going on since last December. A bartender also said tourism is way down too because many are cancelling their vacations due to the seaweed which is a huge drain on local economies. One expat I met informed me this is a once in every 50 year event so who knows the exact reason for the outbreak.

As others have said, I think beaches facing the east are being hit due to the currents while beaches facing west (Negril) are spared.