Up the wooden gangplank in a single-file line, nearly an entire Indigenous village squeezed onto the Aquidaban’s front deck. The Tomárahos had taken the boat downriver to vote in Paraguay’s national elections, and then had slept outside for four days, waiting for the Aquidaban to take them home.

Now, more than 200 of them squatted on overturned buckets, crowded on hammocks and sprawled on the floor. No one was quite sure how many life jackets were aboard, but just about everyone was sure the Tomárahos outnumbered them.

“Ever since I was a kid, there was always the Aquidaban,” said Griselda Vera Velazquez, 33, a craftswoman in the Tomáraho village, where there is no road. She regularly takes the boat to medical specialists 400 miles away for her daughter with Down syndrome. “We’re isolated,” she said. “We have no other way.”

Nearby, four cattle wranglers drank beer after beer, tossing empties into the river, on their way to a monthslong shift in the fields. A mother of six, on a getaway after a divorce, balanced on a deck rail, shouting into a video for her Facebook friends. Upstairs, a young Indigenous couple cradled their 17-day-old daughter on the long journey home from the hospital.

For 44 years, the 130-foot white, wooden vessel has been the only regular ferry service to reach this deep into the Pantanal, a floodplain larger than Greece, traveling 500 miles up and down the Paraguay River Tuesdays to Sundays, delivering everything from dirt bikes to newborns. Its bottom level is a floating supermarket, with 10 vendors hawking produce, meat and sweets from the same benches they sleep on. The ship’s canteen is the only place where many communities can find a cold beer.

But as vital as the Aquidaban has been for locals, particularly the Indigenous, to travel more freely through their forest home, it is also a crucible for the cultural hash that has long been a trademark of Paraguay. This landlocked nation of seven million in South America has for generations attracted a steady parade of zealots, idealists, utopians and outcasts from abroad. And for decades, the boat was one of the few places where all these groups mixed.

On board are Mormon missionaries and Mennonite farmers, Indigenous chiefs and Japanese chefs. Mothers breastfeed toddlers in hammocks, farmers tie chickens to deck rails and hunters sell headless capybaras.

But now the boat’s journeys may be coming to an end.

Paraguay has been carving new roads across its remote north, part of a project to construct a transcontinental corridor, from Brazil to Chile, to connect the Atlantic and Pacific Oceans. Those roads and others have cut into the Aquidaban’s cargo sales, and the family behind the boat says business is sinking.

“There are so many broken parts and no money to fix them,” said the ship’s co-owner, Alan Desvars, 35, standing on the front deck in a German thrash-metal shirt. “This is possibly the last year.”

The Boat

The Aquidaban is loud and filthy. The food suspect. The crew grumpy. The mosquitoes ravenous. And by day four, the air is thick with the smells of perishing produce, livestock and ranch hands returning from months in the bush.

To the Desvars, a family of shipbuilders, it is their pride and joy.

The Desvars got their start selling wooden canoes along the river nearly a century ago. Eventually the younger generation realized that the far-flung riverside communities needed more than just canoes. They needed everything.

So they constructed a vessel shaped like a long shoe, made of wood from the pink Lapacho tree and powered by an old Mercedes truck engine, and named it the Aquidaban after a nearby tributary.

It was an instant hit. After it launched in 1979, the crew sometimes had to kick people off at ports to keep it from sinking.

Since then, the Aquidaban and its roughly 10 crew and 10 vendors have traveled the river 51 weeks a year — some for more than 25 years.

“It’s like a family,” Mr. Desvars said. “There are those whom you get along with better. And those whom you sometimes want to kill.”

A tour takes just a few minutes. The cavernous storage well is packed with cases of milk, oil tanks and televisions. Odd-shaped items — mo-peds, a mirrored armoire, a goat — go on the deck. Inside, vendors sell bananas, frozen chickens and deodorant.

The four toilets dump straight to the river — while the showers next to them pump the river water in.

Upstairs, eight cabins with bunk beds offer privacy for those who can pay. The boat fare is $19 for the full river trip; a cabin is an extra $14. Most passengers sleep on hammocks, on benches or on the floor.

Otherwise, they pack the canteen. The cook, Humberto Panza, mostly makes two dishes — rice with chewy bits of beef or pasta with chewy bits of beef. The ample fresh produce downstairs is not on his menu. “I only cook meat,” he said.

The canteen is also probably the Pantanal’s hottest bar.

When the Aquidaban pulled up to one village on a Friday evening, a throng of young Indigenous people pushed their way on. They spilled out of the canteen into the hallway, drinking cans of 69-cent Brazilian beer and smoking cigarettes under “No Smoking” signs. In a village without electricity, it was the town bar, they said — for a 45-minute stop every Friday night.

The Influencers

The Tomárahos were being followed.

Nathan and Zach Seastrand were headed to the group’s village to film what they called the Tomárahos’ “rain dance.”

“It looks like something straight out of Indiana Jones,” Nathan Seastrand said, as he and his brother polished off bowls of Mr. Panza’s stew.

The Seastrands arrived in Latin America from Utah years earlier — as Mormon missionaries. Then, they were clean shaven and wearing neckties and name tags that said “Elder Seastrand.”

Now they were bearded, longhaired and often shirtless social-media influencers who had attracted hundreds of thousands of followers as two beer-swigging, Spanish-speaking “gringos” who venture into the jungle.

“Dude, like a lot of people have talent,” Nathan Seastrand said. “But they don’t have the balls or the recklessness or the stupidity.”

As missionaries, they baptized more than 30 people into the Mormon Church. Then they came across an online analysis that laid out inconsistencies in Mormon teachings. “It was like an anvil on my head,” Nathan Seastrand said.

They left the Church, and began posting online. Think shirtless pics holding anacondas. Now they were filming a documentary on Indigenous groups they planned to submit to the Sundance Film Festival. The Tomárahos were one of their last missing pieces.

The Tomáraho chief drinking beer on the deck, Nestor Rodríguez, said they were the fourth set of foreigners to take the Aquidaban to the village over the past two years. “They’re doing a positive project to support the community,” he said.

The Seastrands said they had gotten the message that they would have to pay for access.

Under a full moon, the Aquidaban pulled up to the village. For 20 minutes, the Tomárahos shouted at one another while looking for their belongings in the dark.

On the edge of the chaos stood the Seastrands. “We don’t know where we’re going,” Nathan Seastrand said.

The Missionaries

In addition to carrying flour, live pigs and tractor parts, the Aquidaban has also been used to spread the gospel.

For decades, missionaries have relied on the boat to reach the hard-to-reach Indigenous communities along the river.

Its northernmost stop, Bahía Negra, is home to perhaps the Mormon faith’s most remote church. As the Aquidaban pulled up on a recent morning, townspeople massed at the river’s edge, awaiting the weekly arrival of their floating grocery store. Among them were two young men wearing neckties, the current Mormon missionaries, placed there, they said, through divine intervention.

“One of the apostles looks at our face, sees our papers, reads a little information about us and looks at a map,” said A.J. Carlson, 18, from Fort Worth, Tex. “Then they receive a revelation.”

Down the road, a group of Chamacoco Indigenous women were weaving baskets in the backyard of their bungalow. “Before them, there was no church. Just shamans,” Elizabeth Vera, 64, said of the Mormons. “Then the Americans came.”

She motioned to Mr. Carlson: “He’s a messenger of Christ.”

Back on the Aquidaban, Emilia Santos was traveling from her Indigenous village to a different church. She was the head cook at a jungle outpost of the Unification Church, the religious movement founded by the Rev. Sun Myung Moon, a Korean man who claimed to be a new Christian messiah, drawing millions of followers — and accusations of brainwashing and bankrupting much of his flock.

The settlement, at Puerto Leda, was made up mostly of Japanese missionaries, so Ms. Santos had learned how to make curries and sushi. She was on her way to start another two-week shift, she said, “always via the Aquidaban.”

The settlers tend to taro-root crops and 20 fish ponds. They have also converted some Indigenous neighbors.

Jamby Balbuena, an Indigenous worker who helps farm the fish, was in the Aquidaban’s canteen drinking beer, on his way to a shift at the settlement, where alcohol is banned. He said he converted two years ago: “I like their religion, following God, all that.”

The Police and the Prisoner

Derlis Martínez looked nervous. The 25-year-old federal police officer in camouflage reliefs and combat boots was transporting his first prisoner, on the crowded boat.

In a tank top and handcuffs, Agustín Coronel, 37, looked relaxed. “He’s my bodyguard,” he said, smiling.

The two had been traveling together since Bahía Negra, where Mr. Coronel had been arrested after hitting his wife. “I was to blame,” he offered, unsolicited. Mr. Martínez had to get him to a court hearing downriver — a journey of nearly two days.

“I can’t sleep,” Mr. Martínez said. “I have to guard him.’’

Mr. Coronel said he would stay awake, too, to keep his travel partner company.

So the two men talked — about Mr. Coronel’s violence and remorse, about hobbies, about life. Back and forth they passed a dried cattle horn filled with tereré, a cold mate popular in Paraguay, sipping from the same silver straw. And side by side they ate at the canteen, Mr. Martínez using his own money to pay for Mr. Coronel’s dinner.

By 2 a.m., after 20 hours together, Mr. Martínez was on a bench downstairs, his bleary eyes on Mr. Coronel, splayed out on the floor, cuffed hands overhead. They had formed a bond, the prisoner said.

Mr. Martínez hesitated. “It’s my job,” he replied.

By morning, they were back in the canteen, admitting they had dozed off next to each other outside the engine room. How were they doing now? “Spectacular,” Mr. Coronel replied.

In the long hours and tight confines of the Aquidaban, Mr. Martinez confessed, “we started a friendship.”

Laurence Blair contributed reporting from aboard the Aquidaban.