The last time I was in Manzanillo, the beaches had a black film where the water met sand. I was told that it was volcanic elements washing up. Is this true, or is it maybe from the filth of the shipping routes and/or leaks from oil tankers? I'd like anyone's take on the situation and if you might have a reasonable geologic explanation for this? I'm just wondering if it's OK to swim in the area as my kids will be coming this time..