Discuss the contrast that Twain creates between the river and the shore.
Explain how the places differ, what each represents, and how the contrasting elements contribute to the overall meaning of the work (theme).
To me, the river represents a refuge for Huck and Jim, especially, from the shore, which represents the society as a whole. The society of the time is bad for both of them -- it wants to enslave Jim and it wants to civilize Huck. On the river, they can escape these problems.
On the river, nothing bad really happens to them. On shore, among other things, Huck's dad can get him, the Grangerford feud can happen and Jim can be taken as a runaway slave.
The contrast between the two contributes to one of the overall theme of the work, which is freedom. Both Huck and Jim want to be free, each in his own way. The differences between what happens to them on the river and what happens on the shore highlight that struggle.