"we must always light the lantern whenever we see a steamboat coming..."
See in text (Chapter XII)
This lantern, like the fire Huck left on the island and the lights of the ferry-landing Huck floated past in Chapter VII, becomes a symbol of life. Sources of light can be seen as signs of life, which reveal both a person's whereabouts (as with the lantern) and the beauty and power of nature (as with the bolt of lightning).
See in text (Chapter XVIII)
A split log or a piece of timber lain on the ground in place of flooring. This usage is specific to North America and originated in the late 17th century. Twain uses it here to emphasize that this is the country and that the citizens of this town have made only the barest effort to build a clean, respectable place of worship. The pigs lazing about on this puncheon floor are symbolic of the townsfolk, who seek comfort and not personal salvation in the church.
"but I hadn't any clothes on, and didn't mind..."
See in text (Chapter XX)
Huck seems to prefer being in nature than wearing clothes. Here, his nudity is a symbol of his freedom and further characterizes him as an easy-going person who's more at home when he's alone or on a raft than when he's in a city. This kind of behavior wasn't uncommon in Huck's time and could be seen in people who lived on the shores of the Mississippi.