Homer was a self taught artist and based his work on direct observation. He was skilled at watercolor and often painted marine settings. His watercolors often had the boldness of a painting done in oils.
In this painting, you can clearly see the contrast of the sea and the sky, which makes a very dramatic effect. The waves look like they are actually moving and you can see the sky clouding over. The stark white areas on the painting make the painting pop and create a more realistic portrait. The horizon line makes the boat look like it's headed back to shore because of a possible storm or other reason. I think that the contrasting light hand dark colors really make this painting stand out.