In Conrad's "Heart of Darkness," the immediate effect of the second paragraph is one of vastness. Conrad writes:
The sea-reach of the Thames stretched before us like the beginning of an interminable waterway.
"Interminable" means having or seeming to have no end. Vastness is before them. As he looks into the vastness, the narrator sees the sea and sky "welded together" and "the barges drifting up with the tide seemed to stand still in red clusters of canvas...." A "haze rested on the low shores that ran out to sea in vanishing flatness." The speaker's eyes are directed outward toward the sea, the vastness, and the air is dark above "Gravesend" (italics mine). Farther back the air is "condensed into a mournful gloom, brooding motionless over the biggest, and the greatest, town on earth." The yawl is leaving the biggest, greatest town on earth, and going into the vastness of the interminable sea.
I suggest that the mood is one of vastness and foreboding. They seem to be leaving the known, and traveling into the unknown.
To me, the mood of this paragraph is gloomy and foreboding. The total flatness of the scene, with everything looking the same, makes the reader feel sort of dull and lethargic and unhappy. The words "mournful" and "gloom" are really quite descriptive of the whole tone of the paragraph.
This is appropriate for this story because nothing much good happens in it. One of the main themes is how people can become lonely and alienated. This causes them to start to act based on the darkness that lives at their cores. That sounds like a story that deserves a gloomy and foreboding beginning...