Albert Barnes on Matthew 14:28-31

And Peter answered, etc. Here is an instance of the characteristic ardour and rashness of Peter. He had less real faith than he supposed; more ardour than his faith would justify: he was rash, headlong, incautious, really attached to Jesus, but still easily daunted, and prone to fall. He was afraid, therefore, when in danger, and, sinking, cried again for help. Thus he was suffered to learn his own character, and his dependence on Jesus: a lesson which all Christians are permitted to learn by dear-bought experience.

(m) "if it be thou" Php 4:13