On another occasion where Jesus was teaching a large crowd of people next to the Sea of Galilee, we were told that Jesus asked his disciples to have a small boat ready for him in case the crowd got too large (Mark 3:9). We can thus assume that he was in this same boat when he was now teaching the crowds (Mark 4:1).
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.