Although I moved away a long time ago I was born and raised near St John's. Still go back home quite often.
You will be very pleasantly surprised by St John's. It is the oldest city in North America and most certainly one of the most beautiful. The harbor is very small, your ship will probably dock close to Water Street where all the tourist stores can be found. Very unique and colorful buildings. George street is also very close (most bars per capital in the world). If you decide to take a tour you at least be taken to Signal Hill and Cape Spear.
You should have time for a tour and walk around.
St John's is a wonderful old city. Also, Newfoundlanders are know as extremely open and friendly. No aggressive people here like we find on many cruise stops.
You will love St John's. I've been on many cruises and haven't seen anything to match it yet (yes I know I am biased). Enjoy.....Jim.