Peter trusted Jesus enough to jump out of a boat on a stormy sea and attempt to walk on not just water, but violently churning water. Good move, that. Trusting Jesus, that is, rather than over-thinking it.
But then he sees what he has stepped into and becomes overwhelmed. If you reflect on it a little while, it makes no sense. Yet we do this ourselves all the time. We know that Jesus is God the Son, second person of the Trinity that created the waves under our feet. Yet we fear. We seem not to realize that we are in effect saying that God had enough awesome power to create the sea and its powerful waves, but not enough power to keep them from swallowing us whole. How absurd! Do we really think God could create something so powerful that he no longer would have the power to keep it from harming us? Do things get away from God that way? Are we silly?
Our lack of trust is the only thing capable of sinking us.