You really need to contact the cruise line before you sail. Most if not all lines will permit this.The issue is whether or not there will be customs officials available for "permanent disembarkation" versus just "clearing the ship" to dock and allow people off to do tours, etc. Your cruise line would know and probably assist you in doing this if it is not always a regular service.
Not sure if disembarking early without notice would cause a fine to the cruise line(and since they have your credit card they would pass it on to you).
But I don't think you can bypass customs and immigration in Halifax when you were outside of Canada because you are Canadian.
You can ask if they will reduce your cruise fare by disembarking early or at least not charge you for gratuities after you leave so it could save you some money!