if we take the ferry to east boston, can we take it back to the museum?

Yes. Try the little electric water taxi... but not on a windy or choppy day... it would be like riding a bucking bronco!

Yes, the water taxi is available to take you back and forth from the ICA and the Watershed. Please make sure to check the schedule to make sure you can catch a ride back across before the water taxi (which I believe is around 5PM).