But when he saw how strong the wind was he became afraid

"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!"  (Matthew 14: 29 - 30)

But when Peter how strong the wind was he became frightened.
Consider things that tend to frighten us and diminish our faith:

But when he saw the doctor's report he became frightened.
But when he saw the bank statement...
But when he saw the weather report...
But when he saw the poll numbers...
But when he saw the pink slip...
But when he saw the statistics...

Peter took his eyes off Jesus.  Whenever this happens in our lives, we begin to sink into fear.  Peter wisely and providentially applies the antidote to fear which is prayer.  "Lord, save me."

"Have no anxiety at all, but in everything, by prayer and petition, with thanksgiving, make your requests known to God.
Then the peace of God that surpasses all understanding will guard your hearts and minds in Christ Jesus."  (Philippians 4: 6 - 7)