She is due to give birth by mid-July and made the journey from London by helicopter for the naming ceremony of Princess Cruises' 3,600-passenger vessel Royal Princess.
A musical extravaganza was staged ahead of the christening of the new vessel by the Duchess and blue carpet, not the customary red, was laid out for her arrival.
Dressed in a black and white animal print dress and black hat, the Duchess took her place on a stage in blustery conditions that threatened rain and seated opposite in a large grandstand were guests associated with charities that have the Duke and Duchess of Cambridge or Prince Harry as their patrons.
Entertainment included performances from The Band of her Majesty's Royal Marines Portsmouth, accompanied by the Pipers of the 1st Battalion Irish Guards along with pop singer Natasha Bedingfield, who sang her hit Unwritten.
Alan Buckelew, president and chief executive officer of Princess Cruises, gave a speech praising the Duchess for taking part in the naming ceremony.
He said: ''This is, indeed, a very proud day for all of us at Princess Cruises, because it marks the culmination of several years of designing, building and delivering this spectacular ship.
''And we're delighted to name our new ship in Southampton, because our company's ties to Britain are strong.
''To welcome Royal Princess into our fleet, we're incredibly honoured that HRH the Duchess of Cambridge will officially name the ship today, marking the start of its many journeys through the world's oceans and to fascinating shores.
''We can think of no more fitting godmother for our magnificent new ship. Her Royal Highness is an inspiring ambassador for Britain, and she is admired around the world for her style and grace, and for her compassion for others.''
The Right Rev Tim Dakin, Bishop of Winchester, blessed the ship, delivering a prayer before Kate cut the rope which sent the bottle of champagne bottle crashing into the side of the hull, to a large cheer.
The Royal Princess embarks on its maiden cruise to the Mediterranean on Sunday and will sail those waters throughout the summer until it begins a series of eastern Caribbean cruises in the autumn.
Features of the new ship include a top deck, glass-bottomed walkway extending 28ft (8.5m) beyond the edge of the ship, the largest pastry shop at sea and balconies on all outside staterooms.
The vessel is 141,000 tons, has 19 decks and can carry passengers at a speed of 22 knots.
Catherine, Duchess of Cambridge names cruise ship Royal Princess in Southampton
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