National Express. You can book on line. Depending on your age they do half price discounts for 60+ Sometimes the buses do not go into the cruise docks but most times when the ships are there they do.
Just search National Express Coaches put in your dates and it will tell you the times and Prices. It wont cost you much to get the taxi from the bus terminal if its not stopping at the cruise dock on your dates. Hope this helps book early sometimes they have great offers on prices. They have been doing 1p fares from some places.