We were anchored in Salinas, Ecuador waiting for the right time to leave for Easter Island and the rest of the South Pacific. Salinas has a magnificent beach, no rain, and almost perpetual sun. In the four months or so we were there it only rained twice. The first was a minor little shower. The second was an incredible downpour which the locals said hadn't happened in years. When we awoke the morning after the storm the entire bay was a sea of foam like that pictured in the link. I had seen very small examples of this foam in the two salt ponds in the Caribbean - where salt was extracted from the evaporated sea water.
Our assumption was that the huge swath of sand beach had become super-saturated with salt in that arid climate, and the combination of wind and rain had whipped up that condensate like egg whites are whipped. It was an incredible sight.