I've yet to get to Alaska, but, based upon other wildlife/cruise experiences.... 24-105, rent a 400DO, take a monopod and a backup body, done. Batteries, chargers, and a checklist for all gear. I also bought a Peilican case, which was worth it more than I thought.
The reason is size and weight with protection.
The 100-400 I do own, and although tempting, I found it to be used almost always at 400 -- Landscapes, although they always sound so tempting, don't get to print as much as long glass shots seem to, for me. As for why 24, I sold all the glass wider than 24 after noticing not only was it just rarely used, but when it was used the results I could print were no better than 24 anyway. And when I really want that wide pano, I just stitch now.
You may ask why no tripod - but for shooting stars at night with a remote it was a major PIA and very rarely used.
Have fun!