I went whale watching earlier this year, from Monterey, California. My main observation: things can happen REALLY FAST with whales. What you're hoping for is that they'll end their 3-or-so minute dive with a spectacular breach, and if you get one, it can be in any direction around the boat, and is over very quickly.
I had my XF 55-200 on, and actually kept it pretty wide most of the time, around 65mm, and kept it up near eye level. Frankly, many of my shots were "hip shooting", spray and pray, as I didn't have time to get the camera to my eye.
Maybe things are more leisurely on an Alaska cruise, and whales are just fussing around on the surface - or maybe cruise ships won't attempt to get as close as these dedicated whale watching boats, so you'd need a lot more lens. But on my trip, I wouldn't have been able to make use of anything beyond 150mm. I missed a few great moments even sitting there at 65mm, but I'm sure one would get better with practice and experience.
These are two of my best shots, of a Humpback who showed off for us - heavily cropped, but the Fujis have plenty of pixels. This is with the XT-10, and I did feel some pain from the more limited buffer size when going full auto; an XT-1 with its bigger buffer size would have been better.
In any case, I hope you have a great time, and capture some wonderful memories!
