Chaos on the sea was widely understood in the Jewish world as a sign of God's judgment or God's absence. We see it in the creation account, in the flood and in a lot of other imagery throughout the Old Testament.  But, then Jesus rebukes the wind. He speaks to the sea. And it listens! If a raging sea was a sign of God's judgment, then Jesus was presenting Himself as one who had the authority of that judgment! If the sinking boat was seen a sign of God's absence, then Jesus was announcing His presence in way they could never forget!