One of the remarkable things about this miracle of Jesus walking on the water, and the disciples being rescued from the wind and landing strangely safe at their desired haven, is that nothing is made of it in the rest of the Gospel. We are in the middle of John 6, and the entire chapter is devoted to unfolding the implications of the miracle of the feeding of the 5 thousand. So that miracle gets a whole chapter of attention and explanation. This miracle gets none.
So John is not telling this miracle as a way of setting the stage for a long dialogue about Jesus' ability to walk on water. Instead this miracle is embedded in the story of the feeding of the 5 thousand. This miracle in John's mind served that story. John is telling the short and amazing incident of Jesus' walking on the water to clarify or underline something in the story about the loaves and fish.
Here's a clue what that might be. So far as we know, the people that Jesus walked away from in John 6:15 and some of whom he will now spend of the rest of this chapter talking to—they never hear about this miracle. Jesus doesn't talk about it. The disciples don't talk about it. So who is it for?
It's for the disciples. And for us. Jesus is showing something to the disciples that underlines the point of the feeding of the 5 thousand. One of the minor points that we said nothing about last time was that when the 5 thousand had eaten their fill, Jesus said to his disciples in John 6:12, "Gather up the leftover fragments, that nothing may be lost."