In its 15-year history American Stevedoring has moved hundreds of thousands of televisions, loaded tons and tons of coffee and shipped and received everything from sofas to ambulances. Now it is hauling its most precious cargo yet: tenors.

A 69-year-old former oil tanker moored in the company’s container port in Red Hook, Brooklyn, has been transformed into the set for an opera, “Il Tabarro,” by Puccini. For more than two months, dozens of singers, actors and crew members have been commuting to Pier 9B on the Buttermilk Channel to rehearse. The one-act show, a romantic thriller about a longshoreman and his philandering wife, was scheduled to open last night and will continue tomorrow and next weekend.

The action is meant to happen on a barge on the Seine in Paris, and for verisimilitude, this production is hard to beat. Not only is it staged on the tanker, the Mary A. Whalen, with the industrial waterfront of New York as a backdrop, but some American Stevedoring employees serve as extras. (It’s a stretch: they play longshoremen.) The audience watches from the pier a few feet away.

“I’m always envisioning opera happening wherever I go, in city streets or alleyways or abandoned buildings,” said Judith Barnes, the founder of the Vertical Player Repertory, the indie opera company behind the show. Since 1998, Ms. Barnes, a soprano who plays the wife, has been staging classic and new operas in the company’s regular home, a former sculpture studio in Cobble Hill.

Last year it performed “Il Tabarro” there, to good notices. But that theater seats only about 60, and the stage is tiny. Given the opera’s subject, and the company’s proximity to the Brooklyn waterfront, Ms. Barnes thought, Why not do it on a boat? The nearby Gowanus Canal proved unsuitable, so she got in touch with Carolina Salguero, the director of PortSide New York, a nonprofit company that owns the Mary A. Whalen and promotes the Red Hook waterfront.

Ms. Salguero, a former photojournalist who was so bewitched by the area that she abandoned her career to run PortSide, agreed to participate, mainly because the subject was a natural fit. “It’s a waterfront-themed story,” she said. “If it weren’t, as wonderful as it is, it wouldn’t be here.”

Both Ms. Salguero and the owners of American Stevedoring, which provides a free berth for the Whalen, were eager to help, as a way to remind New Yorkers that there is a working port in their midst. In addition, said Matt Yates, the company’s director of commercial operations, “it’s such a cool idea.”

Of course, there were the inevitable challenges in transferring the production from land to water. “I frankly had no idea how much work was involved,” Ms. Salguero said. “I thought, they’d performed it before, they’ll bring it here, and we’ll just plop it in.”

Instead she found most of her summer occupied with preparations for the show, whether it was moving the boat an entire ship length to create proper sightlines (using time- and effort-consuming forklifts because Vertical Rep could not afford a tug), wiring it for lighting, adjusting the gangplank or explaining the tides. American Stevedoring pitched in, emptying a warehouse to create dressing rooms and lending personnel; their employees gave the actors a tutorial on how to convincingly sling the lightweight burlap sacks they use as props.

But two days before opening night, to-do lists still filled several dry-erase boards in Ms. Salguero’s office aboard the ship. The lights began working only halfway through the final dress rehearsal. The 17 orchestra players had trouble hearing one another; ditto the cast. And the professional stevedores were M.I.A.

“They don’t have the same approach to rehearsing as we do,” Ms. Barnes said.

Still, some parts of the production married beautifully: both the story and the show begin at sunset. A lower-level deck doubled as a neat orchestra pit, with room enough for the timpani and an upright bass. The metal warehouses that line the dock bounce sound, so the acoustics are surprisingly good. (No amplification is used.) And the setting served as an inspiration for the director, Beth Greenberg. Because the ship was built in 1938, Ms. Greenberg and Ms. Barnes decided to move the action to 1940s Brooklyn. (À la Brando, the men wear white undershirts.)

Though the libretto already included some activity on the docks, Ms. Greenberg and Ms. Barnes updated and expanded it. Instead of a traditional song seller, there is a man hawking vintage records; the harpist has been transformed into a ponytailed guitarist who plays 1940s music. And other waterfront characters wander through: detectives, cigarette vendors, hookers.

“I thought about a wharf and the kind of nightlife that might appear in that world,” said Ms. Greenberg, who is moonlighting from her job as a stage director for New York City Opera, where she is a 19-year veteran.

“Puccini would’ve loved it,” she added.

The cast had to adjust to the boat’s swaying, but even that proved well meshed. “You’re feeling the motion of the ship, and the music begins with this lovely rocking,” said Robert Lewis, a longtime Vertical Rep member who is in the show’s chorus. “It almost provides a choreographic experience for the performers.”

Samuel Fye, 29, a fourth-generation longshoreman, Red Hook native and first-time actor, quickly volunteered to be in the show, but found that, in fact, it was stretch. “The acting part is harder than the real thing because you’ve got to pretend like you’re lifting something that’s heavy that you’re used to lifting for real,” said Mr. Fye, who normally hoists 150-pound sacks. “You’ve got to make it seem natural.”

But he thought the show was worth the effort. “There’s a lot of negative things that go on over here,” he said, “and this is something positive for the community.”

For Ms. Barnes, who grew up in Brooklyn Heights, it was a way to connect to the bustling waterfront she remembered. “I’ve always wanted to get out to the piers and see what it was like out there,” she said. “Wanting to stow away — as a kid, that was my dream.” Now she hopes that this will be the first of many site-specific productions.

“I’m looking at this kind of alleyway near the Gowanus that has fire escapes and bridges connecting two buildings, and thinking, what kind of opera could we do out there?” she said.