Just did an Alaskan cruise last month with Princess Cruises... Sailed out of Seattle and made stops at Juneau, Skagway, Ketchikan, Tracy Arm Fjord, and Victoria (BC)... I'm sure yours maybe similar... With that said, I tried to bring gear that would cover both reach and width... So I packed the following in my backpack (Lowepro CompuRover AW)...
Camera: 1D4 and Canon G15 (P&S)
Glass: 400 4L DO IS and 16-35 2.8L
Teleconverters: Kenko Pro DGx 1.4x and 2x
Filters: Hoya Pro Digital CPL, Lee ND's and GND's
Support: Gitzo CF + RRS BH-55 + Wimberley Sidekick
We had a mini-suite (with a balcony) stateroom and it helped alot with shooting landscapes while the boat was maneuvering from port-to-port... There were instances where I'd capture sea life (whales, birds, sea otters, killer whales, dolphins) from my balcony... But the excursions is where you get up close and personal with the wildlife...
I never had an issue with being too close @ 400mm... NEVER!!! So your 100-400 should suit just fine... You'll probably be using the longer end, most of the time anyways... Hehehehe... The 16-35 2.8L was perfect for everything I need, width wise... Granted I had a 1.3x cropped body, that didn't hinder it one bit... It was the filters that brought the images to the next level... If you don't have any (CPL, ND, and/or GND), I highly recommend you invest in a set... Truly brings all the details, tenfold!!!
Anyhoo... Here are a few examples from last month's Alaska Cruise Vacation...

IMAGE LINK: http://www.flickr.com/photos/airbutchie/9652205183/
Sunset Shipper...
by
airbutchie
, on Flickr
IMAGE LINK: http://www.flickr.com/photos/airbutchie/9669664883/
Eagle's Watch...
by
airbutchie
, on Flickr
IMAGE LINK: http://www.flickr.com/photos/airbutchie/9661565681/
Sunrise at Sea...
by
airbutchie
, on Flickr
IMAGE LINK: http://www.flickr.com/photos/airbutchie/9647319164/
Double Plumes!!!
by
airbutchie
, on Flickr
IMAGE LINK: http://www.flickr.com/photos/airbutchie/9714776009/
Serene Sunrise...
by
airbutchie
, on Flickr
If you have any specific questions, please feel free to PM me...
