I have been on many Juneau whale watches, the listings are above of the averages. Mid Sept is a very iffy time to go. Last 2 seasons were exceptional with great weather, but highly abnormal. Usual for that time frame is average 3 days of rain, temps 40-50's, cold, damp. Necessary is to be well prepared for the worse case. Early Sept, is better but again preparation is the key.
Mid Sept has dusk by 7pm so plan port time carefully. Possible northern lights- be out from 1am on clear nights for a look. Sometimes, you can get a bridge call?? Inquire if this is possible. For any "animals" you need tours.