The Coast Guard helped clear protesters who briefly disrupted the second voyage of Hawaii's first-ever passenger ferry service among the Hawaiian islands, but the fight over the ship's environmental impact is far from over.

The Hawaii Superferry made two packed trips Sunday — two days ahead of schedule — after the state's Supreme Court ruled last week that the state should have required an environmental report before the ferry launched.

A dozen protesters on surfboards blocked the ship for more than an hour from docking at Kauai's Lihue harbor Sunday, but the Coast Guard cleared them away. Hundreds more protesters onshore beat drums and chanted in support of the surfers.

The protesters object to the ferry's failure to conduct an environmental impact study, saying its plan to ply 400 miles of Hawaii waters each day endangers whales, threatens to spread invasive species and will worsen traffic and pollution.

Environmentalists said they planned to ask a Maui judge Monday to halt the ferry, which is the first alternative to air travel among the islands where 1.2 million people live and tens of thousands of tourists arrive each day.

Hawaii Superferry Inc., in a statement late Sunday, said it was disappointed by the delay on Kauai.

"We are sorry to see that minority dissident groups have chosen to oppose a service that the people of Hawaii have overwhelmingly embraced," the statement said, noting it had sold 22,000 tickets.

The state said that since there was no injunction against the service, it wouldn't be stopped.

Loaded with people attracted by a special $5 fare for the 100-mile voyage past scenic island shores, the Superferry completed its maiden run to Maui and arrived back in Honolulu on time Sunday, then began its trip to Kauai. Passengers lauded the air-conditioned ship, equipped with snack bars and high-definition TV screens, for its luxury and convenience.

On both trips, the $95 million ship was loaded with more than 500 people and 150 cars, the limit placed on initial voyages. It's built to carry more than 800 passengers and 200 cars.

After Sept. 5, the fares will go up significantly: Round trips from Honolulu to Maui or Kauai, with taxes and a fuel surcharge, will cost more than $240 for one passenger and a car.

The environmentalists' attorney, Isaac Hall, said he will seek an injunction to prevent the Superferry from doing business until environmental studies are completed. Such reviews typically are required for projects that use state money and land, such as harbors, and they can take months or years to complete.

Superferry supporters say the ferry is being treated unfairly because other harbor users such as cruise ships didn't have to go through extensive environmental reviews.

"These standards should apply to all players, not just the newest, the smallest and the most popular," said David Cole, the chairman, president and CEO of Maui Land & Pineapple Co., which invested $1 million in the ferry.

Superferry officials say the ship's water jet propulsion system means there are no exposed propellers to strike aquatic animals.

Before Sunday, the only way to travel among the Hawaiian Islands was with highly competitive local airlines now engaged in a fare war.

Superferry Chief Executive John Garibaldi, who mingled with passengers, said one reason for the service was to avoid a repeat of the effect of the nation's grounded airplanes after the Sept. 11, 2001, terrorist attacks. Then, island residents and tourists dependent on air travel were stranded for several days.

Austal USA, which built the ferry in Mobile, Ala., is also building a second scheduled to serve the Big Island starting in 2009.