Chuck:

Unfortunately, all of the vessels you list carried 8" of one configuration or another. Is the shell round or oblong? If it's oblong, it's probably a rifled shell from one of the ironclads.

Also unfortunate, for your case, is the fact that the Deck Logs you asked about were seldom very specific about the locations where they fired their guns, unless it was at a named point on the river or at a target on land owned by someone who was fairly well known (i.e. "Farmer Davis").

Can you be more specific as to the location where the shell was found?