Huck had gotten scared because of the fog and the noise. After he had found a place to rest and passed out. Then he wakes up and doesn't remember where he is. He sees the river and something floating in the middle and starts to paddle towards it. He finally finds Jim, he is sleeping. Huck wakes Jim up and Jim starts to panic, he though Huck was dead. Jim explained to Huck what had happened during the night; the fog and them getting lost. Huck couldn't remember any of it and though Jim was drunk.
Maybe its a good thing that Huck can't seem to remember what had happen during the night. I would be scared about me being alone in dark, not only in the river but fog coming towards you and something from behind. I would probably would want to forget about that. Jim in the other hand, I feel sorry for because he had thought that Huck was dead and he was all worried, and again he thinks Huck's ghost is haunting him. Well at least they found each. :)