Peter said to him in reply, "Lord, if it is you, command me to come to you on the water." He said, "Come."
Peter got out of the boat and began to walk on the water toward Jesus. But when he saw how (strong) the wind was he became frightened; and, beginning to sink, he cried out, "Lord, save me!"
Very often in the Gospels people address Jesus as "Lord". This title testifies to the respect and trust of those who approach him for help and healing. At the prompting of the Holy Spirit, "Lord" expresses the recognition of the divine mystery of Jesus. In the encounter with the risen Jesus, this title becomes adoration: "My Lord and my God!" It thus takes on a connotation of love and affection that remains proper to the Christian tradition: "It is the Lord!"
-- CCC 448