to be above reproach; to have God-like abilities
—She’ll never disagree with her father. She thinks he walks on water.

Jesus did many miracles, to help people and to show his power. One of his most famous miracles was walking on the surface of the Sea of Galilee:

Immediately Jesus made his disciples get into the boat and go on ahead to the other side, to Bethsaida, while he dispersed the crowd. After saying good-bye to them, he went to the mountain to pray. When evening came, the boat was in the middle of the sea and he was alone on the land. He saw them straining at the oars, because the wind was against them. As the night was ending, he came to them walking on the sea [walking upon the sea], for he wanted to pass by them. When they saw him walking on the water they thought he was a ghost. They cried out, for they all saw him and were terrified. But immediately he spoke to them: “Have courage! It is I. Do not be afraid.” Then he went up with them into the boat, and the wind ceased. They were completely astonished. . . .  (Mark 6:45-51)

In today’s usage, walking on water has become the ultimate example of miraculous abilities. It is often used in a mocking way, to point out that someone does not deserve the adoration given to him.