I would have been scared before he revealed himself because at that time when I see someone walking on water I would be thinking about the five w's.
Once he would have revealed himself I would believe him because walking on water is a miracle something that only can be done by God's power. I would praise him for who he is and how powerful he is.
What would I do if this happened to me? I have no idea I would be amazed, and scared. This event can only be explained as the power of God.