Royal Caribbean's Icon of the Seas sets sail from Miami with room for 8,000 passengers.
The ship is built to run on liquefied natural gas - or LNG - which burns more cleanly than traditional marine fuel.
When it comes to carbon emissions, Royal Caribbean says its new ship is 24% more efficient than required by the International Maritime Organization.
But its methane emissions pose a problem.
Industry experts call it "methane slip" - when engines leak methane into the atmosphere during the combustion process.
"It's actually a step in the wrong direction..."
Bryan Comer is director of the Marine Program at the International Council on Clean Transportation, an environmental policy think tank.
He estimates using LNG emits over 120% more life-cycle greenhouse gas emissions than marine gas oil.
"LNG isn't a short-term or long-term solution, because the moment that you start using LNG as a marine fuel, including in cruise ships, you actually start to emit more greenhouse gas emissions than if you had done nothing and continued to run on the fuels that we're using today."
In terms of global warming effects, methane is 80 times worse over 20 years than carbon dioxide.
Anna Barford is the Canada shipping campaigner at Stand Earth, a nonprofit organization.
"Methane is coming under more scrutiny. We had the International Maritime Organization, which regulates shipping around the world, come together and over the summer announce their greenhouse gas reduction strategy. It does explicitly include reducing methane emissions."
According to the Cruise Line International Association, of the 54 ships on order in the next four years, 63% are expected to be powered by LNG.
Royal Caribbean says it will use different fuels as the market evolves with an ultimate goal of net zero.
