We just did this on our May 24th Alaska Cruise. We booked it through their website (only because Disney charges Adult prices for 11 year olds)...we saved so much money. The crew is so friendly, and our kids had a bunch of hands-on time with the various sea creatures. They tell a bunch of stories, and it is very educational...makes you appreciate eating crab more. They took us close to this island and started throwing herring off the side of the vessel and about 100 or more eagles just showed up and it was amazing.
Like the previous poster said, you just walk down to Berth 3, and you board the vessel. We were one of the first families on and we got front row seats. They have heaters that they will turn on if it gets to cold, and they will pass out full length fleece lined jackets to anyone who wants one. Can't express enough how courteous and friendly this crew is.