Huck longs to be out in nature, because it represents comfort, calm, ease and freedom. When Miss Watson takes him in and tries to "civilize" Huck, he hates it. He states in chapter one that
"it was rough living in the house all the time...she put me in them new clothes again, and I couldn't do nothing but sweat and sweat, and fell all cramped up."
When he occasionally gets out in the woods again, he says that it makes him feel "free and satisfied." Mark Twain, through Huck, uses nature to symbolize freedom. It is through the river that Jim and Huck are always able to escape and be happy with themselves, and find beauty. It is only when they encounter civilization in its many forms that they are unhappy.
Civilization in the story is used to represent degredation, poverty, sloth, laziness, violence, chaos and unhappiness. Huck is uncomfortable being civilized, and only happy out by himself nature. On the raft, whenever they go into a town, it is filled with crooks, dirty people, and they often encounter pretty terrible violence and depression too. They encounter lynchings, tarrings, murders, theft, suicide, depression, and many other unsavory situations. There is not much about civilization to recommend it, and Twain uses Huck's free spirit to satirize many of the less ideal aspects of civilization at the time.
I hope that those thoughts helped; good luck!