Yes, there is historical evidence that it was something that was performed--same as the firing of the cannon from the river boat. It was a widely held belief in Britain and the U.S. An April 1767 story in the The Gentleman's Magazine from London describes a similar scenario where the body of a drowned infant was found using a loaf of bread with quicksilver. This story on the Historical Society of Pennsylvania website details that one as well as several other published examples where loaves and mercury were used to search for drowned bodies: https://www.hsp.org/blogs/hidden-histories/looking-for-the-drowned-dead-with-a-loaf-of-bread-and-mercury.

As for the cannon, there is a New England Journal of Medicine from 1869 (volume 81) where the question is posed to the editors, noting "Every now and then, the quiet of the good city of Boston is disturbed by the firing of a cannon over water where a drowned body is supposed to lie..." and asking if there is any scientific basis to it. (Spoiler alert: Their answer is "No.")

I don't know if this works as a source, but this story from the New York Times (http://www.nytimes.com/1990/04/17/science/q-a-392990.html) details the rationale behind quicksilver loaves and the use of cannons. For the quicksilver, it was "based on hopeful etymology concerning the biblical 'bread of life' and 'quick' or 'living' silver, so called because of the flowing form of mercury." And the use of cannons was based on the thinking that the concussion would make the body's gall bladder burst, which would somehow cause the body to float. (The question writer to the New England Journal of Medicine alludes to the gall bladder as well.)

I would also like to add to this the "rattlesnake will find its mate" thing: where Huck kills a rattlesnake and puts it in Jim's bed to scare him, and the snake's mate finds it and curls up with it.

Was this something that people actually believed happened? Some weird folklore or old wive's wisdom?