Peter had seen the power of Jesus enough times to know that he should expect the unexpected. Peter knew that unusual things happened when Jesus was around. He probably wasn't that surprised to see Jesus on the water, once he realized who it was. In this regard, Peter was no different than the others in the boat. All people make decisions based on past experiences.
All people are capable of expecting certain outcomes based on past patterns.
When Peter stepped out of the boat, he was making a decision based on what he had experienced. He was walking as much on faith as he was on water. The water is in the story only for dramatic effect. Jesus could have just as easily walked to the boat through the air. Peter probably would have reacted the same way.