After she lost track of her son on the ferry she looks in the water, and the foam on the water has the shape of a skull.
The seaport where the two friends say goodbye is not the one at El Hierro. The producers chose instead to film the scene in Agaete seaport, Gran Canaria (a completely different island to the East), due to the overwhelming feeling the hight cliffs and steep mountains in the background provide, in line with the film's desolate atmosphere. The actual port at El Hierro would have disrupted that feeling, being more populated and placed in an area of gentle slopes.
The Atlantic island of Hierro is the smallest of the Canary Islands, an archipelago off the coast of Africa under the jurisdiction of Spain.