This language to describe the sea is actually really interesting and creative. Think about what the sea looks like when you are standing on the shore: the waves have some amount of height (a few inches to several feet) and they vary in color a bit as the edges of the waves (the foam) would be kind of gray or white, and the water would appear dark. But if you think about what waves would look like when standing ABOVE then from a great height, you realize they would lose their three-dimensional quality and would appear flat and wrinkly like old skin. This description of the sea does two things: it reinforces the solitary height on which the eagles stands as he surveys all his surroundings, plus wrinkles suggest old age and perhaps weakness, which then contrasts with the strength and thunderbolt-like power of the eagle.