In this incident, the disciples of Jesus when they were left alone in the boat and struggled the whole night in troubled weather, might have looked for help. They might have longed that someone would come and help. After the whole night of struggle, in the morning they found someone walking towards the boat. It is less likely for a ghost to be there in the sea in the dawn. They would imagine the impossible as happening but still would avoid the possibility of God being able to intervene in their situation.
Faith is to believe in divine possibilities.
That is to say faith is allowing the possibility that God could intervene in our situation. Doubt is denying this possibility. Faith adulterated with doubt is full but impotent because it always have other alternative than God. There is another remarkable incident in the life of Jesus. This time the hero is a Roman centurion.