What great replies you have already received! The "Alaska" topic grabbed my attention immediately. We are heading back for 9 days in July!.
Let me echo and expand on some of the advice that you have already received:
1)
ND graduated filters .... yes yes yes ... make sure that you read Ron Neibrugge's blog, he mentions them in several places. But just as important, get yours early and use them so that you understand how they work. - I personally use Lee filters, they are great to contact and ask questions. Check out their site here:
http://www.leefilters.com/index.php/camera/ndgrads2)
The advice about wearing your pack was "outstanding" ..... Not much worse than feeling like you are "lugging around" your camera gear. I would suggest wearing your backpack for more than an hour at a time. A minimum of 2-3 hours a few times. Sometimes a pack is just not a good fit for a person's size or comfort requirements ... often times a bag will feel better by just rearranging gear and better distributing the load inside the bag.
Rain is always a possibility so I would suggest a bag that has a rain cover feature. If you are the "So what if it's raining, I'm getting the shot" type .... a rain cover for your camera might be a good investment.
http://optechusa.com/rainsleeve.html3)
Circular polarizer ..... Yes yes yes
You will be on the water so in my opinion a polarizing filer is a must, and not a cheap one. On your cruise you will possibly have opportunity to photograph whales, dolphins, seals, otters, etc. An inexpensive filter can be worse than no filter. Remember filters are a piece of photographic glass.
I would think there is a circular polarizer discussion or several here in UHH .. try searching for them.
Like the ND grad filters ... practice with the CP filter before your trip so that you have a comfort level using it.
4)
Image storage? ... I see that you mention laptop or ipad, and you mention extra memory cards. The following is just me (some might say that I am anal about this but this method has saved my bacon a couple of times) - Each evening (or midday) I download my cards to my laptop, in folders, to allow sorting and help me "remember" later. Then I copy the downloaded images to an external storage device. Sure I have double copies but things happen, cards fail, computers crash, get stolen, etc. I store the external drive in a different place (piece of luggage from the laptop). Think "trip of a lifetime".
I am a big fan of Western Digital My Passports. They are storage devices and not an external hard drive, so they are easier to use in my opinion. They have several sizes up to 1TB and won't break the bank.
http://www.amazon.com/Passport-Essential-Portable-External-Midnight/dp/B0041OSAZS/ref=sr_1_1?s=electronics&ie=UTF8&qid=1357047142&sr=1-1&keywords=WD+My+Passport+Essential+SE+500+GB5)
Where does your cruise dock in Alaska? Seward, Whittier or another port? Will you have time for a small boat cruise to the glaciers? There are several quality cruise tours to Kenai Fjords and the glaciers. They are various lengths of time .. 3, 4, 6 hours, etc. It is possible to get much closer to the glaciers and really experience them ... also a much lower perspective to the water when compared to the large cruise ships ... oh yeah and wildlife up much closer. Just a thought. Here's a suggestion
http://www.kenaifjords.com/?gclid=CIH4uOulx7QCFQ0GnQodwm8AHQ