You get up to Piazzale Roma which is at the start of the Grand Canal and catch a Vaporetto down the canal to St Marks Square. Its €15pp for a 24hr pass. Alternatively you can take a private water taxi that will charge you approx €8.
I would ask Princess if they are arranging anything...we got a water bus (arranged by Thomson) when we were there last year which took us to the area just along from St Marks Square. Can't remember price but I'm sure it wasn't much.
Princess operate water buses for its passengers. They depart next to the ship and drop you 2 minutes walk from St Marks Square. Cost $15 per person from the tour desk on board and last for 24 hours and they operate until 1.00am.
The water taxi from the ship is great and u will find yourself at st marks square area. they charge 15 dollars each on board and u can use it as many times as you like. if u return for a emal on board emerald then leave enough time as it takes approx 20 mins back to the ship but it took another 25 mins tog et off as they were filling the boats and emptying but only have one boat at once. but otherwise a great service think we got on at 5.30 evening to come back from st marks and eventually got off at 6.15..........w :)e should have been in eveening meal by then but we eventually got to meal at 6.35 (big ship) waiter was ok as we ate fas.
Interested to hear that the pass from ship to San Marco is a 24 hour one, and wonder if it could then be used for the day of arrival, in this case the 11th May, after 1 p.m. until the day of departure 12th May, to go ashore after disembarkation as an independent traveller, before 1 p.m. that second day.