Two views of why Jesus taught the crowds on the beach from a boat:
Jesus now teaches at a distance from the crowds that gather around him in hope of healings and physical contact. Once he is in a boat, they can no longer touch him to receive healing, but they must listen to his voice. The repeated pauses in the narrative heighten the sense of a necessary choice. Jesus turns to his disciples, not as instruments of healings or miracles but as true believers, just as he had demanded in relation to his own family (Matthew 12:46–50), allowing only the believers to find him again in the house. Finally, at the end of the day, Jesus departs, leaving the crowds with only his words. The Word itself should be enough for them.
The boat gave Jesus a place to sit, and it was customary among the Jews at this time, for a master to sit while his students remained standing (see Luke 4:20).
Interpretation 1:
In Matthew 13:2 we see that Jesus becomes a teacher at a distance from the great crowds that gather around him—drawn by the hope of healings and physical contact. Once he gets into a boat, they can no longer touch him, but they must still reckon with his voice. The healings cease; only his teaching remains. Now the crowds are forced to listen to his teachings. The deliberate pauses in the narrative heighten the sense of a necessary choice. During these intervals Jesus turns to his disciples. He does not use them as instruments of healings or miracles but emphasizes instead the circle of believers—just as he had done earlier in relation to his own family (Matthew 12:46–50)—and allows only them to find him again in the house. Finally, at the close of the day, Jesus departs and leaves the crowds with nothing but his words. The Word itself ought to be sufficient for them.1
Interpretation 2:
The boat provided Jesus with a place to sit, and it had been customary among the Jews during this time, indeed up until at least the time of Gamaliel, Paul’s teacher, for a master to sit while his students remained standing (see Luke 4:20).2
2 And great crowds gathered about him, so that he got into a boat and sat down. And the whole crowd stood on the beach.