This is about when they find a floating house and Huck's father is in it.
Jim does this out of concern for Huck. Huck loves his father; he's afraid of him and he doesn't really want to be around him, but it is his dad.
I think Jim really cares about Huck. He is also very superstitious. In both cases he doesn't think it is in Huck's best interest to see his father's body. It is sufficient that Huck find out without witnessing it. It is unclear if Jim knew the full extent of Pap's cruelty to Huck but Jim's motives were pure of heart. I don't think Huck needed to see the corpse of his father floating down the river. His relationship was strained at best and finding out later would have been sufficient.
"Huck and Jim climb aboard to see what they can salvage and find a dead man lying in the corner of the house. Jim goes over to inspect the body and realizes it is Pap, Huck's father. Jim keeps this information a secret."
In addition; Jim like Huck's company and assistance, but I don't think he did this for any other reason than Huck was in fact too young to be witness to this type of death. I'm talking about the death of a father, which for a boy would be different than the death of anyone else.
