Symbolism, Imagery, Allegory
As a mariner and traveler, the sea plays a pretty big part in Crusoe's life. Whenever a storm hits the ocean, Crusoe is immediately penitent and begs God for help. When the skies are clear and the waves are calm, Crusoe seems to forget all about that religious stuff. For example:
In a word, as the Sea was returned to its Smoothness of Surface and settled Calmness by the Abatement of that Storm, so the Hurry of my Thoughts being over, my Fears and Apprehensions of being swallow'd up by the Sea being forgotten, and the Current of my former Desires return'd, I entirely forgot the Vows and Promises that I made in my Distress. (10)
The ever-changing sea serves as a useful metaphor for Crusoe's fickle relationship with God.