On Tuesday, in a ceremony that, of course, involved a soccer ball, the Argentine soccer superstar Lionel Messi pressed a button and a bottle of champagne smashed against the bow of Icon of the Seas, christening the world’s largest cruise ship at its home port of Miami. Like an A-list celebrity stepping onto the red carpet, the arrival of Royal Caribbean’s 250,800-ton ship has captured the world’s attention, with some marveling over its cutting-edge features, like the largest water park at sea, while others criticize the gigantic ship’s potential to damage the environment.

With the capacity to carry nearly 8,000 people, the 20-deck, 1,198-foot-long vessel — whose inaugural cruise with paying passengers departs Jan. 27 — is the size of a small city. There are eight “neighborhoods” packed with amenities that include a 55-foot waterfall, six water slides and more than 40 restaurants, bars and entertainment venues.

According to Royal Caribbean, the ship, which is registered in the Bahamas, also sets a new standard for sustainability with the use of energy-efficient technology designed to minimize the ship’s carbon footprint and move closer to the company’s goal of introducing a net-zero ship by 2035.

“We live by one single philosophy, which is to deliver the best vacations responsibly,” said Nick Rose, the vice president of environmental stewardship at Royal Caribbean Group. “And to do that we build with the core principles of sustaining our planet and communities.”

For decades the cruise industry has been criticized for its negative impact on the environment. A 2021 study published in the Marine Pollution Bulletin found that despite technical advances, cruising remains a major source of air, water and land pollution affecting fragile habitats and human health.

While environmental groups have welcomed some of the features on Icon of the Seas, like its advanced water treatment system, some say building such huge ships is contrary to the cruise industry’s long-term goals of sustainability and preservation.

“The ships are getting bigger and bigger and that is the wrong direction for the cruise industry to be going,” said Marcie Keever, director of the Oceans and Vessels Program at the environmental organization Friends of the Earth. “If you were really thinking about sustainability and not your bottom line, you would not be building a cruise ship with a capacity of nearly 10,000 people.”

With more than five different brands, Royal Caribbean has a fleet of 65 cruise ships of various sizes. Icon of the Seas was built to meet demand and deliver experiences that its consumers were seeking, the company said, adding that all its ships carry the same sustainability principles of energy efficiency, and advanced waste and water management.

Transitioning to alternative energy sources

Icon of the Seas is Royal Caribbean’s first ship to be powered by liquefied natural gas, or L.N.G., a fossil fuel that the cruise industry has touted as a cleaner alternative to the commonly used heavy fuel oil.

“L.N.G. is currently the fossil fuel available at a scale that has the best performance in reducing atmospheric emissions,” said Cruise Lines International Association, the industry’s trade group, in its 2023 Environmental Technologies and Practices Report, citing analysis from Sea-LNG, an industry coalition that promotes the benefits of L.N.G. as a viable marine fuel.

But environmental analysts are concerned about L.N.G.’s long-term problems. Despite emitting around 25 percent less carbon dioxide than conventional marine fuels, they say, L.N.G. is mostly methane, a powerful gas that traps more heat in the atmosphere over time than carbon dioxide.

According to a 2020 greenhouse gas study by the International Maritime Organization, the United Nations body that regulates global shipping, the use of L.N.G. as a marine fuel grew 30 percent between 2012 and 2018, resulting in a 150 percent increase in methane emissions from ships.

Bryan Comer, the marine program director at the International Council on Clean Transportation, said the reason methane emissions have grown faster than the use of L.N.G. is because ships are switching from steam turbines to dual-fuel internal combustion engines. “They are more fuel efficient, but emit large amounts of unburned methane to the atmosphere in the form of ‘methane slip’ from the engine,” he said, pointing to I.C.C.T. research that predicts demand for L.N.G. will triple between 2019 and 2030, as will methane emissions.

“Even if ships used 100-percent renewable L.N.G. bio or e-fuels, methane emissions from ships would still double between 2019 and 2030 because of methane slip,” he added.

Royal Caribbean says that L.N.G. was the most viable alternative fuel available when decisions were being made about how to build Icon of the Seas more than 10 years ago.

“People will say L.N.G. is not the long-term fuel and we agree and view it as transitional,” Mr. Rose said. “We have built the ship to make it adaptable to future fuel sources.”

The company is preparing to debut the Celebrity Xcel next year, a 3,248-passenger ship that will be equipped with a tri-fuel engine designed to accommodate methanol, which several environmental groups consider to be one of the most promising fuels to achieve carbon-neutral sailing.

Shore power capabilities

When cruise ships are docked at ports, their engines and diesel generators are often running on fuel, emitting carbon dioxide into populated areas. Icon of the Seas has been built to run on shore power electricity in ports, a cleaner alternative to fuel, and hopes to become one of the first cruise ships to plug into the local power grid at Port Miami when shore power facilities are set to become available in the spring.

Three ships can plug in safely and simultaneously at the port on any given day, including Icon of the Seas, a spokeswoman for Port Miami said.

“When it comes to sustainability, there is no silver bullet and we want to pull every lever possible,” Mr. Rose of Royal Caribbean said. “So if we can pull into a port that has cleaner shore-power capabilities we want to plug in so we don’t use any fuel.”

The problem is that most ports don’t supply shore power: Only 2 percent of the world’s ports currently offer it for cruise ships, according to CLIA. Royal Caribbean says it is working with ports and other cruise lines to further its use.

Converting waste to energy onboard

Expanding its 30-year “Save the Waves” program, which aims to help keep trash out of landfills and the ocean, Royal Caribbean has built what it says is a first-of-its-kind waste management system on board Icon of the Seas that converts waste into energy.

The microwave-assisted pyrolysis technology, known as MAP, takes food, biowaste and cardboard waste and turns it into small pellets. The pellets are then heated up to produce a gas that is converted into steam energy that Royal Caribbean said would be used to power the ship’s water park. The system also produces biochar, which has the potential to be used as a fertilizer.

The company said it will have a better understanding of the system’s output while the ship is in full operation in the coming months, but so far it takes around 25 kilowatts of electricity to operate the system with an output of 200 kilowatts.

“It won’t take much energy to run the system,” said Mr. Comer, of I.C.C.T, but, he added, “It won’t produce much energy for the ship, either.”

Advanced water treatment

Icon of the Seas is equipped with an advanced purification system that is designed to treat all wastewater onboard, from toilets and showers to kitchen galleys. More than 93 percent of the ship’s fresh water will be produced on board through a system of reverse osmosis, which removes contaminants from water, the cruise line said.

Ms. Keever of Friends of the Earth said Royal Caribbean deserves credit for the treatment systems. “They’re installing the most expensive and best sewage treatment technology on their ships, and it’s important because they are the biggest cruise line and are showing the industry that they can do it, pay for it and they should,” she said.

Fuel cells

In its 2023 promotional video series, “Making an Icon,” Royal Caribbean said Icon of the Seas would be its “first ship with fuel cell technology,” which would be used to power parts of the ship like the air conditioning and elevators.

But it won’t happen yet.

Fuel cells combine hydrogen and oxygen to make electricity without combustion and their byproduct is water, meaning that they do not emit as many greenhouse gases as traditional fossil fuels. While Icon of the Seas has been built to accommodate fuel cells, the batteries have not yet been installed, according to Bloom Energy, the fuel cell manufacturer working with Royal Caribbean. Because of the size and scope of the project, Bloom Energy said it encountered issues with external suppliers.

Bloom Energy is now focused on solving the issues for larger fuel systems that are being planned for Royal Caribbean’s 5,668-passenger Utopia of the Seas, which is scheduled to enter service next year. Suminder Singh, the vice president of marine at Bloom Energy said the next opportunity to equip Icon of the Seas with the cells may not be for another five years, when the ship is scheduled to go into dry dock. Royal Caribbean says it may not take that long and the decision will depend on the success of the technology on Utopia.

Mr. Comer of I.C.C.T. said that while fuel cells would be a great option, they have similar life-cycle emissions as conventional oil-based fuels if they are made on land using natural gas. “We need hydrogen made from renewable electricity,” he said. “And if we have that and use it in fuel cells, then you would basically have zero-life cycle greenhouse gas emissions.”