I checked, and it was an imaginary ship, the CSS Birmingham (and they also changed the story of course).
The CSS Birmingham was, originally, a Confederate Ironclad warship sunk at the bottom of the sea, until the war god, Ares, raised it from the depths so Clarisse may have transportation and a crew to escort her to the Sea of Monsters. It is not meant for use in deep water.
Here is an example of a confederate ship:
The Navy website discusses the CSS Atlanta (originally the USS Atlanta)
CSS Atlanta, an 1006-ton ironclad ram, was originally built in Scotland in 1861 as the merchant steamship Fingal. In November 1861 she ran the blockade into Savannah, Georgia, with a large cargo of weapons and military supplies. After Union forces closed the exits from Savannah, preventing her further use as a blockade runner, Fingal was converted to an casemate ironclad and renamed Atlanta. She made her first appearance as a Confederate warship in mid-1862.And yes, there are Navy Civil War re-enactments.
Atlanta made two efforts to attack Federal warships blockading the coast and rivers leading to Savannah. The first, in early 1863, was thwarted by obstructions blocking the route to the sea. In June 1863 Atlanta made her second attempt, targeting blockaders in Wassau Sound. There, on the 17th, she encountered the U.S. Navy monitors Nahant and Weehawken. In a brief battle, Atlanta went aground and was overwhelmed by Weehawken's superior firepower, forcing her to surrender.
The captured ironclad was taken into the Union Navy as USS Atlanta, commissioning for service in February 1864.
as for Percy and friends:
They changed the plot for the film, of course: originally they didn't meet in Charybdis...actually, they changed quite a few things about the plot...on the other hand, the ship of the bad guys was not a Princess Cruise liner, but a huge yacht. Budget I guess.
“What’s that noise?” Annabeth shouted, keeping her eyes on the Hydra.
“Steam engine,” Tyson said.
“What?” I ducked as the Hydra spat acid over my head.
Then from the river behind us, a familiar female voice shouted: “There! Prepare the thirty-two-pounder!”
I didn’t dare look away from the Hydra, but if that was who I thought it was behind us, I figured we now had enemies on two fronts.
A gravelly male voice said, “They’re too close, m’lady!”
“Damn the heroes!” the girl said. “Full steam ahead!”
“Fire at will, Captain!”
Annabeth understood what was happening a split second before I did. She yelled, “Hit the dirt!” and we dove for the ground as an earthshattering
BOOM echoed from the river. There was a flash of light, a column of smoke, and the Hydra exploded right in front of us, showering us
with nasty green slime that vaporized as soon as it hit, the way monster guts tend to do.
“Gross!” screamed Annabeth.
“Steamship!” yelled Tyson.
I stood, coughing from the cloud of gunpowder smoke that was rolling across the banks.
Chugging toward us down the river was the strangest ship I’d ever seen. It rode low in the water like a submarine, its deck plated with iron. In
the middle was a trapezoid-shaped casemate with slats on each side for cannons. A flag waved from the top—a wild boar and spear on a
bloodred field. Lining the deck were zombies in gray uniforms— dead soldiers with shimmering faces that only partially covered their skulls, like the
ghouls I’d seen in the Underworld guarding Hades’s palace.
The ship was an ironclad. A Civil War battle cruiser. I could just make out the name along the prow in moss-covered letters: CSS Birmingham.
And standing next to the smoking cannon that had almost killed us, wearing full Greek battle armor, was Clarisse.
“Losers,” she sneered. “But I suppose I have to rescue you. Come aboard.”
no, I have no idea what the wild boar is about, unless it is the Colydonian boar...which is the one that sits over the cabin for the kids of Ares.
considering this is a kid's book/film, the author seems to have done a lot of research to get things right...