Between Wrangell [1] and Petersburg [2], the ferry passes through tortuous Wrangell Narrows, a 46-turn nautical obstacle course that resembles a pinball game played by ships. This is one of the highlights of the Inside Passage trip and is even more exciting at night when the zigzag course is lit up like a Christmas tree. Be up front to see it.
The larger cruise ships are too big to negotiate these shallow waters between Kupreanof and Mitkof Islands, thus sparing Petersburg from the tourist blitz and glitz that most other Southeast towns endure.
Links:
[1] http://www.moon.com/destinations/alaska/southeast-alaska/wrangell
[2] http://www.moon.com/destinations/alaska/southeast-alaska/petersburg