Obviously you are going to Zeebrugge, which is about 8 miles from Bruges. You could do it by tram and train and some ships run a complimentary bus to the train station in Blankenberg (which has a better train service than Zeebrugge). Your best bet is to see if anyone else on your ship has organised a minibus that you cna join as well. Go to
www.cruisecritic.com
and join the site (its free), then go to the 'boards' and look for the 'roll call' for your ship and sailing date. You will find loads of info there and possibly be able to join some private organised tours which are better and cheaper than the ships ones.