Her Serene Highness the Princess Grace de Monaco, decorously ignoring the Bronx cheers of some picketing longshoremen, yesterday smashed a bottle vintage champagne against the bow of the first luxury passenger liner ever to be christened in New York harbor.

To the blare of tugboat sirens and the wail of a quartet of Scottish bagpipers, the princess officially named the Cunard Line's new cruise ship, the Cunard Princess, and in so doing rekindled the hopes financially beleaguered city officials that the long-declining Port of New York could make a comeback.

"I name this ship Cunard Princess - may God blss hr and all who sail in her," said the princess. Then, with surprising force, she smashed a bottle of Champagne Ritz - a special vintage made exclusively for London's Ritz Hotel - against the prow of the sleek, white-hulled ship.

Apart from spraying herself and a gaggle of news photographers with the traditional bubbly, the princess finished the dockside ceremony without a hitch, and was applauded warmly by a gathering by about 500 persons.

Cardinal Terence Cooke, archbishop of the diocese of New York, blessed the ship, which is registered as Cunard's third modern passenger vessel, saying, "May she be a haven for her passengers and may God protect her from the bite of the seas and the wind."

But the 750-passenger, 17,586-ton ship, which arrived here Tuesday from Southampton, England, on its first passage, already has felt the bite of the seas and the wind.

It's master, Capt. Peter Jackson, noted that the new ship was two days late docking here because of "a little problem with the North Atlantic, which was not really in the best of moods."

On Saturday, the ship will begin its summer season of weekly New York-to-Bermuda runs. In the winter, it will sail from Fort Lauderdale, Fla.

Compared to the Great Queen ships owned by Cunard, the Princess is small. It is 536 feet long, compared to the Queen Mary's 1,019 feet, and its 750-passenger capacity is far less than the 1,957-passenger capacity of the Mary.

Although its ceilings are lower than the older liners and its interior seemingly less spacious, the Cunard Princess is more functionally designed and conveniently equipped, according to its owners.

The solemnity of the ceremony was interrupted briefly by a dozen hardhatted longshoremen who drifted away from a demonstration in front of the West 48th Street passenger terminal and briefly heckled the princess as she greeted local dignitaries on an excursion boat moored between the liner and the dock.

The longshoremen had been protesting a decision by city officials to convert one of the Hudson River piers into a pound for illegally parked automobiles. They claimed that by converting the pier, the city took away potential shipping jobs.

May Abraham Beame optimistically said yesterday that the city has cause for "rejoicing" because by basing the Princess here, the Cunard Line "is strengthening our overall maritime position and boosting passenger cruising in and out of our great city."

Although it opened a new $40-million glassed-in passenger terminal, with berths for six ocean liners, New York harbor has been waging a losing battle to regain the preeminence in passenger travel it enjoyed in the late '50s, when the great ocean-going liners of the world regularly berthed here, creating a panorama of multi-colored smokestacks all along the West Side Highway.

Some critics have said the new terminal is in danger of becoming a financial white elephant to a city already heavily in debt.

In 1955, for example, 884,000 passengers sailed from New York harbor, compared to only 373,000 last year. Each year since 1960, there has been a steady decline.

The most dramatic falloff has been in transatlantic passengers. In 1955, there were 700,000 persons on trips to Europe, while last year there were only 37,500.

As jet aircraft replaced transatlantic liner travel, increasing numbers of cruise ships opted for so-called "fly-sail" package tours operating from southern ports such as Miami or San Juan.

However, Thomas P. Ronan, chairman of the Port Authority, yesterday said the Princess christening marked a "new beginning" in luxury passenger liner travel from New York, and predicted that other shipping lines would follow suit.

Cunard officials said passengers on the Princess will be given the option of flying one leg of the New York-Bermuda voyage.