Albert Barnes on Matthew 12:15

Jesus--withdrew himself, etc. He knew the design against his life. He knew that his hour was not yet come; and he therefore sought security. By remaining, his presence would only have provoked them farther, and endangered his own life. He acted, therefore, the part of Christian prudence, and withdrew. Mt 10:23.

Mark adds, that he withdrew to the sea; that is, to the sea of Galilee, or Tiberius. He names, also, the places from which the multitude came; an important circumstance, as it throws light on the passage quoted by Matthew, Mt 13:21 "In his name shalt the Gentiles trust."

Pressed by the crowd, Mk 3:9 he went aboard a small vessel, or boat, called by Mark a ship. This he did for the convenience of being separated from them, and more easily addressing them. We are to suppose the lake still and calm; the multitudes, most of whom were sick and diseased, standing on the shore, and pressing to the water's edge; and Jesus thus healing their diseases, and preaching to them the good news of salvation. No scene could be more sub- lime than this.