Remember, 70f is considered hot for alaska.
I went this year on the 2nd southbound of the year for Radiance of the Seas for the season(May 10-17)so it actually snowed on us a few times and never got above 50f until we got to Vancouver, It also rained almost the entire time. We flew to Anchorage and flew home from Vancouver. Remember Seward and Whittier are somewhat far from Anchorage so plan for transport to the port.

If you want nicer weather i would do July/August.