"Muddy Mississippi." "Ol' Man River." "Proud Mary." And even more, if you want them. The Mississippi River might as well be a national symbol; it's definitely a majorly important symbol for Huckleberry Finn. It represents freedom and possibility—but also, maybe, the problems of a drifting life.
Sure, the river is Huck and Jim's transportation. It's taking them from captivity (slavery; child abuse) to (hopefully) freedom in the state of Ohio. But the river ends up symbolizing freedom in its own right.
Before hitting the rapids, Huck feels confined—both by both society (which, figuratively, kept Huck imprisoned by its restrictive rules) and by Pap (who, literally, kept Huck locked up). And the river is the only route they can take if they want to be free both in that present moment and in their respective futures. Check out the way Huck describes it:
So in two seconds away we went a-sliding down the river, and it did seem so good to be free again and all by ourselves on the big river, and nobody to bother us. (29)
"Free again," "All by ourselves," "nobody to bother us": to Huck, the river represents a life beyond the rules of society. And that's a life he could get used to.
But is freedom all it's cracked up to be?
After all, the rules and laws that people like the Widow and Judge Thatcher lay down aren't just meant to make Huck's life miserable; they're also meant to protect him. On the river, Huck and Jim encounter all kinds of life-threatening situations: burglars and potential murders; losing their raft; missing the mouth of the Ohio River; losing the raft again; witnessing the Grangerford-Shepherdson bloodbath; meeting up with the duke and king; oh, yeah, and losing Jim back to slavery.
As Huck drifts down the river, he learns that freedom comes with great responsibility: the responsibility to decide for yourself how to be a good, moral person.