Only during five months of the year does Alaska's weather make passenger cruising in the famous Inland Passage feasible and enjoyable. High season is June through August, when temperatures are between 60 and 80 degrees in the daytime and cooler at night, and shoulder season is early May and late September.
Although cruising during shoulder times is less expensive and less crowded, shore excursions---especially those involving boats and helicopters---are more likely to be canceled then. Temperatures are lower and daylight hours fewer.
Northbound cruises of the Inside Passage, which most often depart from Seattle or Vancouver, tend to be a little less expensive than southbound cruises, which depart from Alaska's Juneau, Ketchikan, Seward, Skagway and Whittier. (Airfares to Alaska are pricey, though cruises are a good value.)
So many cruise ships now crowd Alaska that it's often necessary for ships to drop anchor some distance out and send passengers ashore by boat. Crowded ports also mean guests don't always get their first-choice excursions. So, when choosing a cruise, ask about these issues and make tour reservations well in advance.
If you prefer, you can also see Alaska from an Alaska Marine Highway State Ferry, which enables you to take your vehicle to other ports. These also offer food and private accommodations for overnight as well as a public area for sleeping bags. They depart from Bellingham, Washington, and Prince Rupert, British Columbia.