I have been to Alaska twice. Early June & Late July. Alaska weather can vary greatly. One piece of wardrobe advice is LAYERS.
I did not have the warmer weather some get in Alaska. Be prepared also for rain. Especially in Juneau. As far as whales we did see them.
There are 2 official formal nights. To be honest you will see many variations. If you are comfortable wearing dress pants and a top that is fine. Some overdress & others underdress. Again, go with what you are comfortable with.
Have an awesome cruise