Welcome to Alaska. First of all, disregard the weather forecasts. You know how wrong the forecasters are in the lower 48. Well, they are even worse in AK.
Secondly, even if "rain" is predicted, it is rarely one of those all-day steady rains. It rains a bit in the morning or afternoon or evening. And of course, it rains pretty much every night I tent camp in Alaska (but the nights I spend in hotels, B&B or cabins, it rarely rains - go figure!).
Actually cloudy weather for your Whittier cruise is probably BETTER. You can see the blue in the glaciers much better in overcast weather than when the sun is brightly shining. And photos tend to be better when there is not direct, harsh midday sunlight.
And weather with Denali bus trips? Please tell me you are not looking at weather forecasts for Denali. There are at least 4 mountain passes you drive over from one end of the park road to the other. Let me tell you a story. I was in Denali NP in early June. I camped at Wonder Lake for 3 nights. I brought a bicycle with me so I could bike to Reflection Pond if Mt. McKinley was visible and I could get a reflection photo with alpenglow (already have that shot, but love it anyway). Long story short, it was cloudy and rainy the entire three days (on and off) so I figured I would bring the bike on the bus to Eielson then bike back at the end of the day from Eielson to Wonder Lake - about 19 miles, mostly downhill. My nephew took the bus back. I was at about mile 80(?) and I had a light sprinkle of rain. Not bad actually since it was rather warm so I hardly got wet. When I got back to the campground, my nephew wondered how I did in the downpour that happened. I said, "What downpour?" Apparently it really rained at the campground but hardly anything where I was just a few miles away.
Moral of the story - plan for rain every day then be glad when it doesn't. Take good rain gear (jacket and pants).
>>Will rain ruin the cruise experience by not being able to step out of cabin
Do you melt when you get wet? Why in the world would you not be able to step out of the cabin just because it is raining? PLEASE don't let a little rain stop you from getting outside on the boat!
>>I currently booked 3 days for Denali? Shall I have a backup plan if it rains every day I am there?
If it rains at Denali, then maybe you shorten your hike a bit and spend more time on the shuttle buses. When the sun is shining and it is hot, the wildlife often lays down and rests - much like we do. They don't like the heat either. So in cooler, cloudy and/or rainy weather you might see more wildlife than otherwise.
But really - take rain gear and go with the flow. If someone can't handle a little rain, unfortunately Alaska is not the place for them.