Peter's sinking while walking on water is connected with little faith and doubt.

Matthew 14:28–31

²⁸And Peter answered him and said, Lord, if it be thou, bid me come unto thee on the water.

²⁹And he said, Come. And when Peter was come down out of the ship, he walked on the water, to go to Jesus.

³⁰But when he saw the wind boisterous, he was afraid; and beginning to sink, he cried, saying, Lord, save me.

³¹And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt?