- Good job
First of all, the water is nicely rendered, what is the subject of this class
Posterizing is a good help in getting grips on a composition, but for accurate copying it is more useful to convert to grayscale, as I have done. Then you see that the beach and the calm water are better not left as light.
I think the main reason of the dissatisfaction is not the rendering, but the underlying structure. The foot of the dunes seem to lie on the horizon in the photo, but much closer in your drawing. Likewise the line of the calm water on the beach looks much closer. That does not correspond with the distance to the line between the surf and the calm water. Unconsciously we have the feeling to look from two points at the same time - disturbing
Keep up the good work