The location where this takes place is the Sea of Galilee. Jesus again begins to give instruction at the edge of the Sea. He had done this before (3:7); also then the instruction happened after certain events in Capernaum (3:1; 3:20). By his formulation, Mark reminds us emphatically of the earlier event. When Jesus acts like this now, it has the character of repetition.1
1 Again he began to teach beside the sea. And a very large crowd gathered about him, so that he got into a boat and sat in it on the sea, and the whole crowd was beside the sea on the land.