These are defining moments in the earthly life of Jesus. Both feeding the 5,000 and walking on water happen in the context of grief. The loss of John the Baptist drove him into the wilderness and he was followed by a hungry mob. I'd have been so mad at the crowd for ruining my retreat! Finally, dismissing them he went to the mountain to be by himself. Walking on water was a simple convenience that allowed him solitude.
The disciples were terrified when they saw Jesus walking on the water because they experienced him in a place and in a way they did not expect. It is disturbing to discover Jesus by surprise. It would be even more terrifying not to find him when you look for him.