Article 1 of the United Nations Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment is the internationally agreed legal definition of torture:
"Torture means any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in or incidental to lawful sanctions."
Your Chinese water torture falls under the psychological aspect, where it places an individual in isolation, with a regular stimuli that prevents sleep, disrupts concentration, and so forth. It's effective because it works on the basis of psychological stability involving sleep, and avoidance of a constant stimuli that is predictable. The waiting for the next drop inflicts psychological harm also.
Tickling certainly can be used to inflict suffering on people, no debate. And it could be called a torture if you want to be technical. But it simply doesn't work on the great majority of subjects, in that they are aware that it is harmless to their long term well being and poses no direct psychological leverage either. Outside of your incredibly ticklish people it's not effective.
An effective torture is subject blind. It works on all subjects with a high degree of success. Tickling fails that by a good bit. So it's not a go to by the professionals that deal such things.
Myriads