We did a month long trip to Alaska earlier this year. A Canon shooter here, but I used my 100-400mm lens a lot for wildlife shots, with as much zoom as I could get. A wide angle is nice for landscapes.
Here is a lucky shot at 370mm:
Obviously, I needed the telephoto. And, there are plenty of pixels to crop in closer to the whale if I want.
A couple others with the same setup- both of these are serious crops:
These were all shot as wide open as possible (f/5.6). I didn't try a converter because I wanted to get as much light as possible. My husband did use a converter and it worked okay as long as they weren't action shots ( he used a tripod).
I also took along a Canon SX50 Superzoom. I found it okay for stationary subjects, but not for any action. This bald eagle was shot at 215mm (1200mm 35mm equivalent) while we were riding in a small skiff.
So, I guess I would say 300mm minimum, longer if you can. But, the ability to handhold is important as well. If wildlife isn't going to be a prime subject, a shorter telephoto will be fine. Enjoy your trip!