Hi there, presuming that the ship docks 'round the back' of Venice - and of course actually docks in Venice & not their alternative in Trieste which is miles away - then the easiest way is to get a day pass for the Vaporetto's (waterbuses)
Check the link Venice Vaporetto Routes - Water Buses - Venice for Visitors
for a traveller's guide to using them.
Have a great time,
Alan & Katrina
Next Cruise: P&O Adonia June 2014?
Last Cruise: P&O Oriana Nov 2013
Golden, Grand(2), Caribbean, Sea(4+1 aborted)
Adonia(4+1), Arcadia, Artemis(2), Aurora(5), Azura, Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(5), QE, Caronia(2), Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam