I haven't sailed Alaska on Carnival, however I have sailed Alaska four times. Some advice I would have is always keep looking around. You never know when you will see some wildlife such as whales, porpoise, moose or bear. If you are bringing a regular camera and not digital bring more film than you think you will need. You will be taking many more pictures than you normally take. When you are in near the glaciers or out on the water have a filter on your camera lense to cut down on glare.
Most important relax and enjoy the beauty of Alaska. You are in for a great cruise.