I have limited knowledge, but hopefully I can be of some help. I found this link that has some info. Honestly, I would go to the bottom of the page on that link and call the number under contacts and ask them whatever questions you have. Or, call Halifax tourism directly. I don’t think this is the Halifax tourist center site, but I think they can give you info and get you to the right place if the other contact number is not helpful.
There was a CAT ferry that was taken over by this group from what I understand, but I think it does not go to Halifax, you still would have quite a bit of driving if you flew into an American airport. The site isn’t loading well for me. Most people might still use the term CAT ferry, so at least you will know what they are talking about. I had heard about a Boston to Halifax ferry/cruise that would be overnight, but I don’t think it ever happened. The only cruises I see are 4–7 days up the New England seaboard.
Hopefully, a New England jelly will have better information.