I would also recommend the 70-300L...nice and (relatively) light and compact. Good reach, great IS and focus speed. You also have a lot of leeway when it comes to cropping your photos since the lens has such great IQ.
I took an Alaska cruise 3 year ago and loved it. Tons of great photo opps and always be on the lookout. I was using my 55-250 then and could have used a bit more reach. I was on the back of the ship one day at one of the ports watching float planes take off and land and just happened to catch out of the corner of my eye a large bird flying nearby. I spun and captured a bald eagle flying right overheard. Wasn't really prepared for it but still got a couple of decent pics of him in flight. Like I said, always keep your camera handy and ready to go as you never know what you will see there.
Have fun...it's an amazing trip and easily one of my favorite vacations! Pack lots of memory cards!
You may want to consider renting a wide angle too. For me Alaska was all about landscapes (i.e. go wide) and wildlife (zoom). Even with my kit 18-55 (on a crop), there were definitely many instances where I wanted a much wider landscape shot. Far reaching mountain ranges and ocean views beg for a wide angle lens.
Are you doing Denali tour? TONS of great wildlife opps on that tour. We captured everything from grizzlies, to Dall's sheep, wolves, caribou, moose, lots of birds, etc., etc. Some much closer than expected. Don't know if it's normal or not, but we were able to see all the big ones and the wolf actually ran right past our bus and stopped within 30 yards to check out something right next to the road.