I grew up around boats. In my teenage years, I helped crew a boat through the Queen Anne straight and up through the San Juan Islands along the western coast of Canada. Through the islands, the water was calm but there were a few crossings where we met with the open sea where waves were more than thirty feet high. That meant that we road a wave way up to the peak, and then we were swallowed up, completely surrounded by water on all sides as we sank into the valley between waves. As a child, I was fascinated but as an experienced seaman, my grandfather was in earnest of crossing the straight safely.
Because of my experience, I have a different picture of Jesus and Peter out on the Sea of Galilee. I can see Peter losing sight of Jesus and only being able to see walls of water all around him. We sometimes fault Peter for having so little faith. But if you were in his shoes, a well seasoned fisherman who knows the dangers well, perhaps his fear would make more sense.
Fear... isn't that what separates us from everything good? I don't mean the kind of fear that teaches you not to get too close to a snake or a cliff. That is respect... not fear. Fear is irrational... and unreasonable. Paul calls it "the spirit of fear" (2 Timothy 1:7). He describes us as being in bondage to it (Romans 8:15). Fear is the enemy. If you want to know Satan's agenda for you... consider your deepest fears. He is driving you by them... driving you away from God's call in your life. Driving you from the good and towards what is evil. Fear is like a cattle prod, it's very effective and evil at it's core.
This is why we are called to take courage. Like in 1 Chronicles 22:13 "Be strong and of good courage; do not fear nor be dismayed." Because fear will drive you away from God.