While I suppose I can admit that there might be a fuzzy line when a permissible interrogation tactic becomes torture, here's a good rule of thumb: If the Mississippi Supreme Court in the 1920s was willing to reverse the conviction of a Black man charged with killing a White guy because his confession was elicited under the procedure in question, we're probably talking about torture.
The case in question is White v. State, 91 So. 903 (Miss. 1922), and the technique, needless to say, was water boarding (then known as "the water cure"). Four years later, in Fisher v. State, 110 So. 361, 362 (Miss. 1926), the court went all the way and flatly described water boarding as "a specie of torture well known to the bench and bar of the country."
Water boarding: too barbaric for the guardians of Jim Crow, but a-okay for the 21st century federal government.
Incidentally, there was nothing particularly progressive about the Mississippi Supreme Court of this era -- it was not an opponent of the system of Jim Crow oppression, or even the idea of coerced confessions generally. Nine years after Fisher, the Court held in Brown v. State, 173 Miss. 542 (1935) that a confession obtained after the (Black) suspects had been physically whipped during interrogation was admissible. This case eventually reached the US Supreme Court in the form of Brown v. Mississippi, 297 U.S. 278 (1936), which reversed and held that physical torture was a violation of the 14th amendment's due process clause. Another fun fact: the prosecutor in Brown was none other than John C. Stennis, who pressed the case forward fully aware that the defendants had been tortured by the police. Stennis would later become the U.S. Senator for the state of Mississippi, and the National Forensics League named its entire model Congress event after him.