Each area of value has a light, middle and dark value. So, this area had to be darker than the value of the rocks on the left side of the painting because I want those to be a middle value, but not as dark as the darkest value.
When I finished this step, I noticed that the darkest rock had a shape that looked like a submarine stuck in the rocks. Planning is the key to a good painting! So, I added a dark shape behind it to disguise the sub. I will do a bit more in the following steps.