It may be somewhat difficult to conceive why Jesus should go into a boat and put off from the shore in order to speak to the multitude; but it is probable that this was a small bay or cove, and that when he was "in" the boat, the people on the shore stood round him in the form of an amphitheater. It is not improbable that the lake was still; that scarcely a breeze passed over it; that all was silence on the shore, and that there was nothing to disturb his voice. In such a situation he could be heard by multitudes; and no spectacle could be more sublime than that of the Son of God - the Redeemer of the world - thus speaking from the bosom of a placid lake - the emblem of the peaceful influence of his own doctrines - to the poor, the ignorant, and the attentive multitudes assembled on the shore. Oh how much "more" effect may we suppose the gospel would have in such circumstances, than when proclaimed among the proud, the joyful, the honored, even when assembled in the most splendid edifice that wealth and art could finish!
[Excerpt from Barnes' Notes on the Bible]