I know that most have been mentioned.., but here's mine.
New Brunswick Deer Island Point. Simply the best in the bay. And down by the point, not across from the old custom station. Love the 'fingers' ~sigh~
Nova Scotia - Wreck of the Letitia, Birchy Head for a shore dive
Newfoundland - Bell Island wrecks. A must see
Gadds Point, Gros Morne Natl Park - a best kept secret. a vertical wall that drops from 30' to over 235'
St Anthony Lighthouse - A little cove off the boardwalk on the left allows easy access to the water, a nice wall dive. Been 110' underwater and could look up and see the clouds.