Any places offering massages on the beach at reasonable prices at either of these places, preferably St. Thomas, as its the only port we dont have an excursion booked?
If you are going to be on St. John you could check out Westin Resort's spa and see what they offer. On St. Thomas you could check out Wyndham Sugar Bay or Ritz Carltons spa services. Do a search for the resorts and you sould find information on their spa services.
These resorts most likely won't be $25/hour like in Costa Maya . I don't know of any beaches on St. Thomas/St. John that have what you described, Maybe in Coki Beach but not sure.