If you do any excursion on an Alaskan Cruise it should be Whale Watching from Juneau . There are several options available, from private charters, to 6-passenger boats (commonly called 6-packs) to boats that carry 12 - 150 passengers. Although you can occasionally see a whale from your cruise ship,  it usually takes a lot of time viewing the open waters just to catch a "blow" from the whale, clearing their air hole, or a tail fin/fluke going in water. Even after hours of viewing the open waters you may not even see a whale. All have money back guarantees (not necessarily 100% refund) to see a whale.  Actually a "money back guarantee" for whale watching in Juneau is something whereby it has been years that operators have had to make good. Humpback whales are increasing their total local population at a rate of 7% per year (or doubling every 14 years).  Of course the guarantee means seeing blows, backs and tails. Breaching, bubble-netting, or Orcas nearby is an infrequent occurrence.


There are numerous pluses and minuses to each type of charter. On smaller boats it is very personalized. On the large boats, there is a camaraderie  among the guests. The staff is adequate, usually having 5-7 persons on each vessel. Many have a snack bar (for a fee), and multiple restrooms. Everyone is there for the same reason and most are usually polite and courteous. A naturalist is usually on board for educational talks along the way. It is less personal, and you may not even get to speak with a crew member directly, depending on the numbers on the boat. The vessel is also less nimble than a smaller craft. 


As to a smaller craft, they can more easily position themselves, but also cannot deliberately approach closer than 100 yards of a whale. The number of persons can be limiting to 4-6 person on many, but various slightly larger Coast Guard Inspected boats are permitted to carry from 12 to 49 passengers. The smaller the boat, the better attention to your questions and general conversation about the area can be achieved. As long as you do your homework on the charter, the smaller charter is tailored to the experienced traveler. The smaller charters typically serve complimentary snacks and beverages.  The ability to maneuver into position for whale sightings and interactions is better on the smaller boats. Don't expect to see a whale "breech" or come out of the water entirely, as these sightings are rare.  It is the blow of the whale, and the roll of the humpback, and the tail fluke waving as it enters the pristine waters that is a memorable sight. 


Check your prices through the tour operators, the cruise ships, and the local independent charters. You'll find many tours, providing those once in a lifetime experience that will also be remembered a lifetime. Some last only two hours on the water, while others are two and half hours, three hours, three and a half hours, and four hours. What really counts is time on the water, not total tour time as about an hour is devoted (round-trip) to ground transportation. Many operators combine their tours with visits (by ground) to the Mendenhall Glacier. Don't forget to do your due diligence if booking an excursion from a private operator, and make sure you know what is and is not included, such as taxes, transportation, snacks, guarantees. Have fun, and learn the phrase, "THAR' SHE BLOWS"