The advice would differ on whether you are berthing at Terminal 1 (usually the smaller ships) on the riverside or 3 (in the Ijmond Harbour). Being Viking Ocean it could be either.
From Terminal 3 basically come out of the terminal, up the 2 dozen or so steps opposite and catch the bus #82 every 30 minutes to Amsterdam Sloterdijk station for trains and trams to Centraal.
From Terminal 1 I personally would walk the mile or so (have done so several times when taking the Newcastle ferry from Terminal 2) to the cross canal ferry terminal to catch the bus #82 there, but most cruisers would probably prefer a taxi. Having got into the taxi you may want to stay in it for the drive all the way to Centraal Station.
Also check to see if Viking are offering a transfer bus into the City Centre.