Pennant-bedecked and fresh paint, the Steamship Authority’s newest ($3.8 million) and biggest ferryboat (230 feet, 1,000 passengers, 494 net tons of freight), the motor vessel Nantucket, docked Wednesday at the Oak Bluffs wharf for public inspection. The Regional High School band and hundreds of cheering spectators had welcomed her to Woods Hole at the end of her 871-mile voyage from her Jacksonville, Fla., shipyard birthplace. She goes into service from the mainland to Oak Bluffs and Nantucket today.