They don't! Generally they go naked! Remember that they are in the deep south and it is extremely hot and there is intense humidity! In several chapters, Huck says that he and Jim swim and lay out on the raft while floating along. In one scene, a wave washes over Huck while he is asleep and he nearly gets swept into the river, but he says he didn't mind because he didn't have clothes on anyway! Having no clothes on puts the two at their base elements--they are purely natural and at one with nature when they are in this state. In civilization they must be clothed and it is in civilization where the two meet with the most trouble.