Take a cab. There are cabs waiting at the dock when you get off the ship. (I'm from Juneau, so I take a cab out to the airport and rent a car).
Take the cab drivers number so you can call him back around. No worries, the visitor center won't let you be stranded either.
FYI, there's nothing that says you have to stay on the viewing platform. You can walk all the way out to the glacier. Really. It's like a beach walk.
The last time we went, we got there hours before the cruise buses did. I was out next to the waterfall, and looked back at all the tourists on the viewing area. Many came out when they noticed us. The locals go out all time, but you won't see them when the ships are in. It's my favorite place ever.