The ferryboat is firing the cannon because of the belief that doing so will bring a dead body to rise to the surface of the water.

Huck, whose peaceful morning silence is broken by this booming sound, realizes it is his body they are searching for. He has feigned his own death so that he could escape from his abusive father in a way that would keep his father from trying to find him. Apparently, the ruse has worked, as a group of people have boarded a ferryboat to look for his corpse.

Huck knows he can't start a fire without attracting unwanted attention. He is hungry, so he goes after the loaves of bread floating in the water. They are blessed, inserted with quicksilver (mercury), then put into the water. The idea is that the mercury will cause them to float right over to a drowned body.

Both of these methods of finding a corpse are superstitions that came to the United States from Great Britain. Neither one will work, which is just as well for Huck, though he never questions the methods, stating them as fact.

Huck takes advantage of the situation to snatch a loaf of the floating bread for his breakfast, noting it is of high quality.

It was “baker’s bread”—what the quality eat; none of your low-down corn-pone.

Fortunately, he takes out the mercury before he eats the bread.