A.T. Robertson Commentary Mark 4:1

"And again he began to teach by the sea side. And there is gathered unto him a very great multitude, so that he entered into a boat, and sat in the sea; and all the multitude were by the sea on the land." — Mark 4:1 (ASV)

Sat in the sea (καθησθα εν τη θαλασση). In the boat, of course, which was in the sea. He first sat by the beach (Matthew 13:1) and then a very great multitude (οχλος πλειστος) made him enter a boat in which he sat and taught. It was a common experience now to teach the crowds on the beach (2:1,13; 3:7-9).

There is gathered (συναγετα). Graphic pictorial present again. See the crowds pressing Jesus into the sea.