In a 1984 JBL article, Elizabeth Struthers Malbon suggested that the boat in Mark’s gospel represents a “mediator” between sea and land, and pointed out that Jesus treats the sea as if it were land (walking on it, showing no concern for the unsteadiness of the waves, etc.).
If we link this to the OT symbolism of sea=Gentiles and land=Israel, we can see the indications of the Pauline theme that Jesus combines Jew and Gentile into one new man. And, the fact that Jesus teaches from a boat shoved out in the sea perhaps gives us an image of the church – the church is a little ark, a little bit of Israel, tossed about on the sea of nations. But there’s no danger, because the Lord of the church walks on the sea as dry land.