Greetings from Canada! As a Nova Scotia native, let me help you out.
There are no ferries travelling from Halifax to Boston. Most
travellers opt to sail from Yarmouth (3 1/2 hours drive south of
Halifax) on one of two ferries -the "Cat", a high-speed catamaran
which travels from Yarmouth to Bar Harbor, Maine, or the Scotia
Prince, which sails from Yarmouth to Portland. You'd obviously be
faced with some driving time on each end to reach your destination.
If you don't have wheels, there are several ways to get to Yarmouth:
Acadian Lines (bus service)
The Cloud Nine Shuttle
Campbell Shuttle Service
A new service, apparently hasn't yet developed a website; can be
reached by phone at (902) 742-6725.
Due to bus/shuttle schedules, you may need to spend the night in
Yarmouth to catch the next boat. It's a great little town supported
mainly by the lobster fishing industry. Here are a couple of links
with information on Yarmouth's attractions, hotels, etcetera:
Yarmouth County Tourist Association
Official Index for Yarmouth, Nova Scotia
To learn more about the Cat and Scotia Prince Cruises, visit
I used the following search strings to answer your question:
"nova scotia ferries"
"yarmouth nova scotia"