Jerome

But when he saw the strong wind, he was afraid. And when he began to sink, he cried out, saying: Lord, save me. The faith of his soul burned, but human frailty dragged him into the deep: therefore a little is left to temptation, so that faith may increase, and he may understand that he is not saved by the ease of his request, but by the power of the Lord.

Commentary on Matthew ↗

Hilary of Poitiers

The fact that out of all those in the boat, only Peter speaks up and asks that he be ordered to come to the Lord on the water demonstrates the force of his will at the time of the Passion. Then coming back alone and following the Lord's footsteps, with contempt for the turmoil of the world as of the sea, Peter is attended by strength equal to disdaining death itself. His timidity, however, gives an inkling of his weakness in the face of future temptation. For though he ventured forth, he began to sink. …

Commentary on Matthew 14.15 ↗

Thomas Aquinas

But seeing the strong wind etc. Here the third help is presented, because he delivered Peter from sinking. And first the cause is presented; second, Peter's petition; third, Christ's help. But seeing the strong wind, he was afraid. At sea the wind does not have a continuous force, nor on land either; hence it was interrupted when Peter entered the sea; but when he was upon the sea, it blew strongly, and then he was afraid. …

Commentary on Matthew ↗