The dead body in the frame house which floats by Jim and Huck is actually Huck's Pap. Jim hides the body's face because he says it's "too gashly" but he is protecting Huck from the pain he may feel knowing his Pap had been murdered.
Mary/Sarah Williams is a slip up that Huck makes when he disguises himself as a girl to find out what's going on in the town since his and Jim's disappearance. He tells his informant that his name is Sarah and then later tells her it's Mary which she immediately recognizes. She concocts "traps" which prove Huck is a boy instead of a girl and he has to make up another whole story to get out of there in order to get back to Jim before he is discovered on the island.
By covering the dead man's face and refusing to let him look at him, Jim shows tenderness and concern for Huck, even though Huck hasn't shown the same kindness to him. Also, Jim knows what Huck does not - that the dead body is that of Huck's father.
Huck goes to shore disguised as a girl, and identifies himself as Sarah Williams to Mrs. Lathrop, whose house he visits. Later, in conversation, he says his name is Mary Williams, a slip which Mrs. Lathrop catches immediately.