There's a massage shop on every Andaman corner. Most are low-key family affairs where traditional one-hour 300B Thai massages are a real steal. Service quality varies and staff turnover is high. Go with your gut instinct or ask locally for recommendations.
If you fancy a more European-style spa experience, book into one of the resort spas spread throughout the Andaman's. Though often affiliated with a glitzy hotel, almost all of these high-class affairs with sumptuous Zen designs and extensive treatment menus accept nonguests. Phuket has the widest choice of upmarket spas, but many other islands have them too.