On most cruise tours you can purchase a meal plan. It works out to about $50/day/pp.
If you do a pay as you go for an Alaskan cruise tour you can figure that you will pay, for two people, around $15 for breakfast, lunches around $20-25 and dinners $25-35.
If you are in a city/town you can go out on your own and find a different restaurant then the ones at the hotel but often they can be hard to find. At places like Denali you basically have to eat at the lodge.