Only accesible by boat. Best place to start from is Auke Bay (about 15 miles North of Juneau). Head west/southwest and then south around the backside of Douglas Island.
Just about any waterway in Southeast Alaska can be a good place to spot wildife, but this area is one of my favorites since it's fairly close to Juneau, and I seem to find Humpback Whales "bubble net feeding" there quite often. Viewable wildlife can also be killer whales, sea lions, bald eagles, even porpoises.