That looks like sargassum seaweed. We saw tons of it in the Caribbean when we were there in 2019. Apparently it is blooming in huge quantites now because of all the fertilizers that are being sprayed in the Amazon and are washing out into the river. The river water runs out into the ocean and is causing this stuff to explode in quanitity. It is choking out many beaches, all the way up to Southern Florida. It's an environmental catastrophe actually! It's so thick that it is causing a lack of sun to reach the coral reefs and it is taking the oxygen from the fish. It's terrible in many ways. However, we scooped it up and made a great garden compost from it. So there is a silver lining if people actually use it for those purposes.