The porch and table with figures creates its own space, echoed in the space of the pier with figures behind it, drawing our eye deeper into the image, to the spars of the docked ship, the buildings and the course of the Thames winding into the distance.
The girl, the captain's daughter of the painting's title, looks in the other direction, counterpointing our attention. We feel that if we just turned our heads we would see what she's seeing.
We're not simply looking at something -- we're inside the painting . . . we're somewhere.