You first need to decide if you want a round trip (i.e. return) cruise or one that goes one way, with you flying in the other direction. The vast majority of these itineraries (both the round trip and one way cruises) are for seven days/nights.

Round trip cruises are offered with Seattle and Vancouver as their start/end points. These cruises only visit southeast Alaska, the long "panhandle" that stretches roughly from Skagway south to Ketchikan. The Seattle departures travel to the west of Vancouver Island, on open ocean, thus reducing the time one can see the lovely "Inside Passage" between Vancouver Island and the mainland, and also exposing the ships to potentially rough waters offshore. The Vancouver departures travel east of Vancouver Island, mainly on protected waters, and are thus more scenic and smoother.

The one-way cruises all depart or arrive in Vancouver, due to US maritime law. The Alaska terminus is either the town of Seward or the village of Whittier, both of which are connected to Anchorage by rail and road. If you choose the one-way option, most people find it desirable to extend their trips by several days (ideally a week or more) in order to visit southcentral and interior Alaska before or following the cruise. Destinations such as Denali National Park, the Kenai Peninsula, etc., can only be reached using one-way cruises.

Most cruisers will tell you that the specific itinerary is far more important than the choice of ships or cruise line. Most of the lines visit the same ports (and in fact some of the small ports of call are rather overwhelmed by several huge ships calling on the same day) but some itineraries will include, for example, Glacier Bay - a stunning scenic area - while others don't include it. (Access is limited to only a few visits a week.)

The message boards at Cruise Critic offer a vast amount of information and opinions about Alaska cruises, ports of call, and departure cities, as well as plenty of comparisons between cruise lines and accommodations. It's a good place to start your research.