If you are lucky then you may get a transfer bus from Piazzale Roma which is where the bus station is (but note - this isn't where the railway ststion is). RCCL should provide details of this. There have been quite a few postings on this board on the very topic but I note that the search facility within Cruise Answers will now only display 1 page of results (even if it says there are 2 or more) when you search a specific topic or keyword (such as Venice). Possibly the site administrator could get this problem resolved (thank you in advance for investigating this.
There is supposed to be a free bus that runs from Piazzale Roma to the port - it never turned up for us. You can walk from PR or you catch a cab (which will be epxensive) or you catch the 6/ bus.