Despite the hopeful nature which some people might see in it, the artist no doubt wanted his viewers to view the painting as a reference to the endless nature of the ocean, and perhaps also the endless nature of the human soul along with it. How did the audience in that historical moment view this work?
As a matter of fact, it is impossible to render the reality in an absolutely perfect way, so every picture, painting, or video shows only one side of it. That argument is what your thesis statement should be. Remember to stick to the topic — no tangents.
Hence, you should be certain that you have gathered all the required information. The body of the essay should be analyzed according to the effect of the composition. Consider the current events going on at the time of the picture's creation and what social or cultural elements or changes may have affected its content.
There are several ways to do this and your assignment may tell you which direction to go. Everybody makes mistakes — revisions let you fix them before they become a problem.