Just to be clear on the foliage in New England (from somebody that lived there for over 60 years). The peak foliage starts in the upper VT/NH/ME region and progresses southward to South Eastward, with Cape Cod being one of the last to reach peak color. This happens over the course of a month or so.
So, foliage will depend on what ports the ship is hitting and the dates.
Mid to late September would most likely give you the best combination of warmer temps and some foliage. Either way you will still get some color as the leaves change, even when it's past peak as you head further North.