During the summer months, you can take a cruise around Boston Harbor on a tall ship with Liberty Fleet. They run several times a day and have two different ships that they sail. Becca and I took the sunset cruise on the larger schooner, the Liberty Clipper.
This was a great, relaxing couple of hours touring the harbor. It was such a great feeling once the ship left the harbor and we lifted the mainsail (really, they as the passengers to help hoist the mainsail) and the captain shut down the engines. All you could hear was the wind and conversation, while enjoying drinks from the cash bar.
The cruise would have been great no matter what time of day, but the sunset cruise was very nice, although it did get a touch chilly once the sun went down. There is also a night cruise, which I suppose, would be good for just drinking and general partying – though the city lights of Boston would probably be very nice. It would probably be very cold out there though.