"Lord, if it is you, command me to come to you on the water." (Matthew 14:28)

When Peter heard Jesus speak to him, “Take courage, it is I; do not be afraid.”He replied loudly with courage, “Lord, if it is you, command me to come to you on the water.”

Peter understood that walking on water was a supernatural phenomenon. However, he might remember that Jesus once said, “Have faith in God. Amen, I say to you, whoever says to this mountain, 'Be lifted up and thrown into the sea,' and does not doubt in his heart but believes that what he says will happen, it shall be done for him.” (Mark 11:22-23)
Jesus was willing to let Peter undergo such supernatural experiences. However, Peter was unable to stand up to the test because his feet were under the surging water. He called out, “Lord, help me!”

This also reminds us that Jesus has stopped the storm, “A violent squall came up and waves were breaking over the boat, so that it was already filling up. Jesus was in the stern, asleep on a cushion. They woke him and said to him, "Teacher, do you not care that we are perishing?" He woke up, rebuked the wind, and said to the sea, "Quiet! Be still!" The wind ceased and there was great calm. Then he asked them, "Why are you terrified? Do you not yet have faith?"” (Mark 4:37-40)

Peter’s faith was tested once again. In this event, he received the best lesson.
As a result of Peter’s willingness to seek for help, he got the chance to walk on the water himself.