Yesterday it was an elderly man in white robes.

But for the hundreds of thousands of pilgrims lining Sydney Harbour's foreshores, catching their first glimpse of Pope Benedict XVI was a life-defining moment.

Sydney's largest cruise ship, the MV Sydney 2000, took the 81 year-old pontiff on a spectacular ''boat-a-cade'' tour of the southern shore of Sydney Harbour.

The short trip from Rose Bay to Barangaroo at East Darling Harbour marked his official entrance into World Youth Day celebrations on ''Super Thursday''. It was a dream winter's day, with still harbour waters and a light breeze as the Pope arrived at Rose Bay pier.

He received a traditional Aboriginal welcome to country by Gadigal elder Allen Madden and a small troupe of Aboriginal dancers clad in kangaroo skins.

The pontiff looked absorbed in the moment as he boarded the boat about 2.45pm, smiling gracefully as he took his vantage point on the middle deck with Sydney Archbishop Cardinal George Pell nearby.

The Pope's vessel glided out into the harbour waters, guarded by a ring of 13 security vessels, including police boats, jet-skis and a tugboat spraying jets of water.

As he greeted selected pilgrims on the boat with him, the light breeze billowed his red elbow-length cape, forced him to remove his small skullcap, and ruffled his white hair.

He moved inside the boat to large lounge chairs and appeared awe-struck as he took in spectacular harbour views.

The pontiff looked particularly taken as the boat passed the Opera House on Bennelong Point and headed beneath the Harbour Bridge.

But these views were nothing compared with the sight of 150,000 frenzied pilgrims from about 170 nations desperate to catch a glimpse of the pontiff as the vessel turned into Darling Harbour.

As the vessel docked at Barangaroo wharf about 3.30pm, the pontiff came on to the boat's deck, bathed in sunlight and smiling broadly at the sight before him. AAP