The ferry is in only its second day of commercial voyages between Honolulu, Maui and Kauai. The Superferry was originally scheduled to start sailings on Tuesday, but the company moved the start date to Sunday and issued $5 one-way tickets to launch the service in advance of a legal challenge by environmentalists.

More than 90 percent of the available space was filled as passengers rushed to experience Hawaii's first modern interisland ferry service at bargain basement prices.

A Superferry spokeswoman said today's Maui and Kauai trips were sold out. The 350-foot vessel can carry more than 850 people and 250 vehicles, but the company projects an average load of 400 passengers and 110 vehicles.

The first day of Hawaii Superferry service to the island of Kauai on Sunday was disrupted by hundreds of protesters on surfboards, swimming in the path of the vessel and lining the docks at Nawiliwilil Harbor, in advance of the courtroom battle over environmental issues today. The U.S. Coast Guard intervened after two hours to clear a path for the Superferry to enter the harbor.

On Thursday, the Hawaii Supreme Court decided that the state was wrong to have exempted ferry-related improvements at Maui's Kahului Harbor from the state's environmental review law.

The Supreme Court ruled on an appeal of an earlier decision by Cardoza dismissing a complaint filed against the Hawaii Department of Transportation by three citizens' groups - the Maui Tomorrow Foundation, the Sierra Club and the Kahului Harbor Coalition. Cardoza had ruled the groups did not have standing to bring the complaint.

The Supreme court ruled that they do have standing and in addition ordered an environmental assessment of the harbor improvements.

Isaac Hall, the attorney for the groups, hopes to expand the case to involve the Kauai sailings as well.

In September 2005, Judge Helen Gillmor dismissed the suit brought by the three groups seeking to require the Superferry to prepare a full Environmental Impact Study, EIS.

The groups filed suit after several months of meetings with Superferry head John Garibaldi produced no agreement for an EIS. Other groups, Kauai County and Maui's Mayor and County Council have called for an EIS review as well, but are not parties to the lawsuit.

What the Supreme Court ordered Thursday is not an EIS, but a less extensive environmental assessment. Even that could take six months to a year to complete.

Concerns range from increased traffic on Maui and Kauai, collisions with whales, impact on campsites and the creation of a new, rapid dispersal method for alien species, the Sierra Club says.

Because the court case involves concerns about impacts only at Kahului Harbor, Judge Cardonza's temporary restraining order does not prevent the Superferry from sailing between Honolulu and Kauai.