The first thing to realize about North/Southbound Alaska cruises is that they are by definition, one-way cruises. So that means that you'll have to book a flight (or make some other arrangements) either to or from the cruiseport.
One-way cruises are also often called "repositioning cruises" which is to say, the cruiseline is repositioning its ship. These trips are often a good value and present you with the opportunity to save time - you can choose to fly back and cover the distance you've just traveled much faster than a returning cruise ship would.
Alaska North/Southbound cruises often travel through the Inside Passage and will also visit destinations such as Hubbard Glacier.
A popular departure point for Northbound cruises (as well as destination for Southbound cruises) is Vancouver. Southbound cruises often originate in Seward.
Northbound Sailings: Celebrity Millennium, Holland America Statendam, Princess Cruises Diamond Princess and Coral Princess, and Royal Caribbean's Radiance of the Seas.
Southbound Sailings: Look for cruise ships like Holland America's Ryndam, Statendam; Celebrity's Millennium, and Princess Cruises' Island Princess.