Absolute fantastic experience. I am not sure I can recommend something else to do in Boston, before you do this. Everything went perfect: Beautiful weather, well equipped boat, and very hospitable and knowledgeable staff. Now for the whales: We saw Seals, Dolphins, Right Whales, Fin Whales, a zillion Minke Whales, but the real highlight was a Humpback with its calf. Absolutely something that I will remember forever.
The trip out there to the feeding grounds is around one hour, probably 25-30 Miles to get there. The boat is very fast, wherefore it gets cold if you are outside.
I went out to see the whales, but also, as a serious amateur photographer to take pictures. So here is some advice to photographers: You need a lens greater than 200mm (I was with 200-500mm on a full frame). You should keep you f at around 11, because these are large animals, so you want depth of field and you'll need a fast shutter ~1/2000, because the whales are really fast.
Unfortunately, some of the action was on the other side of the boat from where I was located; not much to do about that--you win some and you lose some.
The trip was about 3.5 hours, no one really wanted to go back, because there were so many whales to watch, but eventually we got back at around 1:30pm (departure was 10am).