You can show up at the lot without a reservation, the cost per day is more. With that said, it's actually cheaper once you take into account the convenience fee charged by the vendor 🙂
The lot is 5 minutes away. There are two shuttles. The pretty much go every 15 minutes, or once the shuttle is full. It's really not a big deal.
I believe the garage is discouraging cruise parking due to the increase in commercial business in the port requiring parking. There is the lot in front of the ship, but I have no information on it. I cruised twice out of Boston last year and parked in the offsite lot each time. Both times were safe, easy, and no issue. My only complaint, if I had one, is that the shuttle parks all the way at the end of the terminal. So you have to walk the entire length of the terminal to turn in luggage, and then double back to get in line.