The kit lens (18-55) + the zoom (55-250) you have in your bag will cover a lot for you. The 100-400 is nice addition (I really want that one too).
Consider (just to throw a wrinkle to a question you didn't ask) looking at an ultra wide angle lens. You're going up to Alaska with some really impressive vistas, and an ultra wide might be a nice addition to your kit.
Also.....if you are only going to use this big zoom once........consider renting the lens for your cruise.
www.borrowlenses.com (a site I've used) and
http://www.lensrentals.com (a place that gets high marks as well) will rent you the really big gear like the
Canon 400mm f/2.8 IS
Canon 500mm f/4.0
etc.
yes, you won't own the lens after the trip, but if you are only going to use it on this one trip, it might make an option that makes sense. I rented the 400 f/2.8 to shoot a couple of High School football games two years ago and it was awesome.