I interpreted it that the plane dragged the wire in the sea in front of the minesweeper in the heading of the pilot. This would have drawn a line in the sea, as

I interpreted it that the plane dragged the wire in the sea in front of the minesweeper in the heading of the pilot. This would have drawn a line in the sea, as it were. That would be a totally unexpected behavior, which would give the minesweeper's captain the clue that it was a signal to change course to the given heading. The plane would have headed back to the pilot and circled, giving the minesweeper's captain assurance he had understood the signal.