You can't book them until you are on board. The basic 50 min Swedish massage on NCL will run you about $150. It's worth it if you really want one but they are 3×s what my local massage therapist charges, for example. We have some nonrefundable OBC so, for us, massages will be scheduled on our next cruise. After we got lost & wandered around Venice for HOURS on our Jade cruise in 2019, the massage was worth every dollar!