When the Disciples first saw Christ walking on the Sea they were afraid and did not know who it was.
Peter's response, asking the Savior to let him walk on water to know it was him, was showing his doubt and made it seem like he was like others who are always asking Jesus to prove himself.
According to verse 30, Peter began to sink because when he noticed how strong the wind was he became afraid and no longer was able to keep walking on the water.
When he began to sink he ask the Savior to save him and the wording used was that the Savior stretched forth His hand immediately. He didn't hesitate, but was right there.
Doctrine and Covenants 88:67 ---When Peter was able to walk on the water, he was focused on the Savior, having "an eye single to [his] glory". If we fill our bodies with light, so much so that there is no room for darkness or doubt, we will be able to overcome and be unscathed by the storms in our lives. We will come out cleaner, stronger, and more faithful.