d. how the water looks when sunlight shines on it
The stanza states: "I never knew that sun could splinter a whole sea of blue." In this sentence the sun is doing the action "splinter". This tells you that the sun is responsible. The sun is not a fish or a ship, nor does the sun make waves. This leaves your only answer as D. Since the sun also is unable to come down from the sky and physically splinter the water, the speaker is referring to how the sunlight beams down and shines on the water.