I've done 2 of the 7 night cruises to Alaska. They are wonderful if you love scenery and being on the ship. For me, I love one day on the ship and then I want to get out and do something else. The 7 night cruises started with 3 sea days. They only count it as 2 sea days, but it's 3. Lots of people who love cruising were not loving their cruise because it was too cold to hang out and do the outside stuff that they normally do on Bahama cruises. Kids were bored and unsupervised.
It varies year to year. I saw a mom bear and 2 cubs the last week of May on a bus tour in Skagway. Some people saw bears from the train in June at the same port on my next cruise. The bears were far away from the port.
I was told to be careful for bears at Mendenhall Glacier in Juneau in June, but I never saw any there.
Bald Eagles, seals, and whales are pretty much around.
July is the most popular and usually the most expensive time to go. I make the assumption that it must be a great time to go.
If you go in May/June you can have a lot more ice if you are into that. September is supposed to be very rainy. I think July is popular because it's warmer and the bears are more out and active generally.