After having the image sketched out; after the addition of masking tape to where the lighthouse will go; after adding rubber cement to where the whitest parts of the waves will go; do the following:
Sky, mix up a healthy amount of blue with a hint of yellow and green in it, testing your color and value, of course. With a BIG brush and clean water, get the entire sky portion of the paper wet - not soaking, but wet all the same, and then, working quickly, use that same big brush to apply your sky color to the paper.
Aim for big paint strokes and evenness.
Wet on wet helps with the even application, and since there is tape over the lighthouse, you don't have to avoid that area ... Just paint right over it.
Ground, mix up a large portion of brown and yellow.
With your big brush, use the same wet on wet technique to cover all of the ground.
As you can see in the pic, the ground below the lighthouse and the foreground all need to be covered.
Note that the color you test out on your acrap
scrap paper will be lighter on your actual painting because of the wet on wet technique - which has the same effect as adding just slightly more water to the pigment.