frodosmom, that is correct about the Maritime Museum -- you do not need a guide. It's about a 10 or 15 minute walk along the boardwalk. When leaving the ship, turn right.
Anyone who stays on the ship in Saint John is missing out on a lot. As billybuzzy mentioned, the tides are a must-see. You can do this through a ship tour, or take a taxi to the Reversing Falls. I recently read that there is a new harbor walk that leads there -- I will confirm this on my upcoming cruise. There is also a great farmers market in Saint John inside the City Market building, about a 10 minute walk from the pier.
Another idea is to do your own walking tour. This is what we plan to do next time. The Saint John tourism department lists four walking tours on its Web site -- click HERE
and scroll to the bottom of the page for links to the Loyalist Trail, Prince William's Walk, Victorian Stroll, and West Side Walk and Drive.