In a certain city of the Fire Nation, there is a massage parlor. Some would refer to it as a "massage parlor," cordoning it hygienically between quotation marks, but that's not entirely fair; only some of their business is prostitution. Most of the rest is, in fact, perfectly licit massage.
The fraction that remains is illicit massage, a much more complicated proposition.
Effort is put in to keep the secret to Ai-Li's massage a genuine secret. He practices his craft in a dark room, filled with candles and incense to cloud the mind, and it doesn't hurt that his typical clientele are less interested in the massage than what comes after. All things considered, there's very little reason to suspect him of being a secret waterbender.
If someone were to pay attention, though, the Full Moon Ten-Yuan Special might be something of a red flag.