That's the way I heard it, and7barton. Like John says, it was "Simply sawn off". The only place in the United States where the salvors actually took large cast iron cannon apart, that we know of, was at the Mare Island Naval Shipyard, about thirty miles north-east of San Fransisco, CA, established by the U.S. Navy in 1854 with Commander Dahlgren in charge. After the Civil War they had the older, used up and obsolete naval tubes hauled there where huge steam powered gang drills bored 30 to 40 holes simultaneously, from muzzle to breech, in preparation for spliting with wedges, just like so much cordwood. A large bunch of the never used 13" Dahlgren naval guns were split into six equal pieces and hauled off to be melted down and used for bridge parts or some other use. Paul Barnet of South Bend replicas posted a photo of that operation and a description years ago in the Artilleryman Magazine. Don't ask me which issue! I have no idea, I just remember that startling photo and story.
Regards,
Mike and Tracy
P.S. For those not familiar with the Charlestown Navy Yard, it's the same as today's, "Boston Navy Yard". It's located on the Mystic River where it enters the Atlantic Ocean at Boston, MA.