In the house, Jim and Huck find a bunch of playing cards scattered about, whiskey bottles, and a lot of other random stuff. There is a dead man lying in the corner, who had been shot in the back. It appears that it might have been a poker game gone bad. Jim goes over to look at him, and confirms,
"It's a dead man. Yes, indeedy; naked, too. He's ben shot in de back. I reck'n he's ben dead two er three days. Come in, Huck, but doan' look at his face—it's too gashly.”
Huck doesn't want to look at him, and every time that Huck asks Jim about the man, asking questions because he is curious, he tries to change the subject on Huck, saying that he doesn't want to talk about it. Well, at the end of the book, the very last part, Jim confesses to Huck that the dead man was Huck's Pap. Jim hadn't wanted to tell Huck, but he 'fesses up at the end, letting Huck know that Pap wouldn't be coming back any more to bother him, and that Huck would be able to keep all of his money without worrying about it.
That is the largest signifance of that floating house; unbeknownst to Huck, it is where his good ol' Pap died. I hope that helps a bit; good luck!