Is there anywhere on the east coast where one can go whale watching in October?

Massachusetts. Provincetown on Cape Cod and probably other places as well.

We went from Plymouth - you can visit the sights while you're there.

We saw 2 whales until our crew were contacted by a Provincetown boat and the we saw 2 more.

I think the boats in the area work together and I don't think you will be disappointed. If you're in the area for any length of time all of the trips offer a guaranteed sighting.
good luck!

You can also whale watch out of Montauk in the fall. We've done it twice and were successful both times.

Once had a juvenile playing with the boat (popping up first on one side - then the other, lifting it's head up to stare at us and making little water spouts). Mama watched from a distance.

And the boats do radio each other so the chance of sightings is very good - we had a money back guarantee.

The Provincetown trips pretty much end mid-October.

We had family out here to visit a few years ago and we all took a whale watch trip on Oct 31 out of Gloucester to Stellwagen Bank. We were lucky in that the weather stayed warm and the whales had not left yet. From what we learned on that outing, the trips continue if the whales are still there and that is generally around the end of October.