Alaskan cruises are so popular because they are fantastic. Some of the most amazing places in Alaska like Hubbard Glacier and Glacier Bay national park are only accessible by boat. It is quite amazing seeing a giant glacier calving in the ocean and the noise is breathtaking.
Other than the glaciers you are able to see some very nice ports that are rich in Gold Rush and Pioneers history.
It is quite common to spot a lot of wildlife, seals, orcas, whales and bold eagles. The nature and the scenery is stunning, even when the weather is less than ideal.
Alaska Cruises are also easy to catch, they run all summer long from Vancouver, Seattle and Anchorage. They usually only last a week and the ocean is calm and smooth because most of journey is done in the Inside Passage making them perfect for first time cruisers.
The itineraries are either roundtrip from Vancouver or Seattle to Alaska, or they can be north and south bound between Anchorage and Vancouver or Seattle. In peak season it is not unusual to see a ten or twelve day cruise to Alaska from San Francisco.