About the fourth watch of the night: This was about three o'clock in the morning. This Roman way of telling time suggests that Mark wrote for Christians in Rome.
Mark does not explain how Jesus saw his disciples struggling in the middle of the lake at night. It is not important to the story.
Jesus went out to them, walking on the sea: It seems Jesus came to rescue them from the storm. But Mark says that Jesus intended to pass by them. Many have tried to explain this, but perhaps the best explanation is that to pass them by refers to a divine appearance. This is similar to events in Exodus and 1 Kings, where divine glory is shown. The disciples' fear supports this idea, as fear often accompanies divine appearances.
Many have tried to explain the miracle of Jesus walking on water. But Mark, Matthew, and John saw it as a miracle without a natural explanation. If Jesus is truly the Son of God, there needs to be no other explanation.