Legal problems threatened to beach the giant catamaran like one of the whales that environmentalists fear it will run over, so the company moved up the debut by two days.
More than 500 passengers and crew, and 150 cars, were aboard when the four-deck, blue-and-white vessel emblazoned with manta rays pulled away from the dock to a chorus of cheers.
"It's beautiful," exclaimed Stephen Imamoto of Honolulu, who was traveling with his wife and 6-year-old daughter. "I don't like to fly ... You can't beat the price."
Imamoto said he wanted to try the first voyage to see if he gets seasick. Choppy water and strong wind between the islands have scuttled previous attempts at interisland ferry services with much smaller ships.
Inside the 349-foot Alakai built by Austal USA in Mobile, Ala., passengers browsed the gift shop, played cards and ordered breakfast while watching live NFL Sunday football games on high-definition TV screens as the ferry sailed past Aloha Tower.
Before Sunday, the only way to travel among the Hawaiian Islands was with highly competitive local airlines now engaged in a fare war.
Superferry sold out its first voyage in 30 minutes Saturday, offering $5 one-way fares for passengers and the same for cars. More than 400 of those aboard got right back on the Alakai for the voyage back to Honolulu.
The launch, originally set for Tuesday with fares 10 times that much, was moved up after the Hawaii Supreme Court ruled unanimously Thursday that the state should have required an environmental review before the Superferry started service. Three environmental groups have sued, concerned that the vessel could collide with humpback whales, spread invasive species and create long traffic delays.
A dozen protesters greeted the hundreds of enthusiastic passengers after the ferry docked at Maui's port.
Among the protesters on Maui was Joyclynn Costa of Haiku, who held a sign, "Respect our home." She said the company didn't consult with Maui residents about the service, which she said will pollute island waters.
The environmentalists' attorney, Isaac Hall, said he will seek an injunction today to prevent the Superferry from doing business until environmental studies are completed. Environmental reviews are typically required of projects that use state money and land, such as harbors, and they can take months or even years to complete.
Superferry supporters say it is being treated unfairly because other harbor users like cruise ships didn't have to go through extensive environmental reviews.
Superferry officials also say the ship's water jet propulsion system means there are no exposed propellers to strike aquatic animals.
Environmentalists said the company acted in bad faith by launching early and trying to build public support with steep discounts.
More than 19,000 people have signed up to take advantage of the $5 fare through Sept. 5. After that, round trips to Maui or Kauai, with taxes and a fuel surcharge, will cost more than $240 for one passenger and a car-- if the courts don't stop it. A second ferry being built in Mobile, Ala., is scheduled to serve the Big Island starting in 2009.