I'm not sure if there is any month that is better for whale watching in the Gulf of Alaska.
I have a suggestion re Alaska. During the summer of 2007, my wife and I were in Seward Alaska and we went on a whale watching/wildlife/glacier boat trip to the Kenai Fjords National Park. This was an unforgettable trip not only because of the mumerous whales that we saw but the eagles, glaciers and so on. There are a number of companies that provide these boat trips. We really enjoyed the company that is the lead concession for this.
Hope that this helps.