The raft can be seen as a symbol of good fortune, having come to Jim and Huck on the island. They do not have to steal or cheat their way into ownership of the raft. It simply comes to them.
One night we catched a little section of a lumber raft—nice pine planks. It was twelve foot wide and about fifteen or sixteen foot long, and the top stood above water six or seven inches—a solid, level floor.
The raft also serves to symbolize freedom, as this is the primary vehicle for their travel down-river. More specifically, the raft can be seen to stand for collective freedom and mobility.
On the raft, Huck and Jim are able to stay together, living as more-or-less as equals. Often, Huck takes a canoe to go off on his own to perform a task that would be dangerous with the two of them together. The canoe can be argued to represent individual freedom, in this way (or individual responsibility), while the raft represents a more collective freedom.
Gold becomes associated with death by the end of the novel. Huck and Jim acquire gold when they steal the canoe from the criminals on the sinking steam boat. Those criminals die soon afterward.
Also, during the Wilks family episode, Huck hides a bag of gold in a coffin, which is then buried with the dead man.
Though gold may not exactly represent death, it is closely associated with death in these episodes.
The raft symbolizes Jim and Huck escaping oppression. For Jim, he is escaping slavery and Huck is escaping his father and Miss Watson's lifestyle.