Jesus walks on water, Peter’s invited to join him but sinks, after being rescued, Jesus replies with “Oh thou of little faith” again, the water is the word of God, here the law, Christ has not yet been crucified, no man can walk righteously before God in the law.
However, Jesus being God himself and perfect, can walk above the law, or on the water in this illustration. The only way for us or Peter to get to God is by faith in Jesus alone, not in works or in the law! (Matthew 14)