I'm not sure if you can book before you board, However, if you go to the spa as soon as you board, I'm sure you can find an appointment that suits you.
Mary Lou Scanlon
NCL Pride of America April 24, 2010
NCL Epic February 12, 2011
RCCL Allure of the Seas - September 18, 2011
Celebrity Eclipse - February 11, 2012:
RCCL Navigator OTS - February 9, 2013