Wrong. Japanese war criminals were hanged for a long list of things they did, and what we are currently calling "waterboarding" wasn't on the list.

From this document at this website. (the actual water torture method is boldfaced):

Torture and Other Inhumane Treatment

The practice of torturing prisoners of war and civilian internees prevailed at practically all places occupied by Japanese troops, both in the occupied territories and in Japan. The Japanese indulged in this practice during the entire period of the Pacific War. Methods of torture were employed in all areas so uniformly as to indicate policy both in training and execution. Among these tortures were the water treatment, burning, electric shocks, the knee spread, suspension, kneeling on sharp instruments and flogging.

The Japanese Military Police, the Kempetai, was most active in inflicting these tortures. Other Army and Navy units, however, used the same methods as the Kempetai. Camp guards also employed similar methods. Local police forces organized by the Kempetai in the occupied territories also applied the same methods of torture.

We will show how the Chiefs of Camps were instructed in Tokyo before assuming their duties. We will also show that these Chiefs of Camps were under the administrative control and supervision of the Prisoner of War Administration Section of the Military Affairs Bureau of the War Ministry to which they rendered monthly reports. The Kempetai were administered by the War Ministry. A Kempetai training school was maintained and operated by the War Ministry in Japan. It is a reasonable inference that the conduct of the Kempetai and the camp guards reflected the policy of the War Ministry.

To indicate the prevalence of torture and the uniformity of the methods employed we give a brief summary of these methods.

The so-called "water treatment" was commonly applied. The victim was bound or otherwise secured in a prone position; and water was forced through his mouth and nostrils into his lungs and stomach until he lost consciousness. Pressure was then applied, sometimes by jumping upon his abdomen to force the water out. The usual practice was to revive the victim and successively repeat the process.

There was evidence that this torture was used in the following places: China, at Shanghai, Peiping and Nanking; French Indo-China, at Hanoi and Saigon; Malaya, at Singapore; Burma, at Kyaikto; Thailand, at Chumporn; Andaman Islands, at Port Blair; Borneo, at Jesselton; Sumatra, at Medan, Tadjong Karang and Palembank; Java, at Batavia, Bandung, Soerabaja and Buitennzong; Celebes, at Makassar; Portuguese Timor, at Ossu and Dilli; Philippines, at Manila, Nichols Field, Palo Beach and Dumaguete; Formosa, at Camp Haito; and in Japan, at Tokyo.

Torture by burning was practiced extensively. This torture was generally inflicted by burning the body of the victim with lighted cigarettes, but in some instances burning candles, hot irons, burning oil and scalding water were used. In many of these cases, the heat was applied to sensitive parts of the body, such as the nostrils, ears, abdomen, sexual organs, and in the case of women, to the breasts. We have evidence of specific instances in which this form of torture was employed in the following places: China, at Hankow, Peiping, Shanghai and Nomonhan; French Indo-China, at Haiphong, Hanoi, Vinh, and Saigon; Malaya, at Singapore, Victoria Point, Ipoh and Muala Lumpur; Burma, at Kyaikto; Thailand, at Chumporn; Andaman Islands, at Port Blair; Nicobar Islands, at Kakana; borneo, at Jesselton; Sumatra, at Palambang and Pakan Baru; Java, at Batavia, Bandung and Semarang; Moluccas Islands, at Amboina; Portuguese Timor, at Ossu; Solomon Islands, at Buin; Philippine Islands, at Manila, Iloilo City, Palo, Bataan and Dumaguete; and in Japan, at Kawasaki.

The electric shock method was also common. Electric current was applied to a part of the victim's body so as to produce a shock. The point of application was generally a sensitive part of the body such as the nose, ears, sexual organs or breasts. The evidence shows specific instances of the use of this method of torture at the following places: China, at Peiping and Shanghai; French Indo-China, at Hanoi and Mytho; Malaya, at Singapore; thailand, at Chumporn; Java, at Bandung, Buitenzorg and Semarang; and in the Philippines Islands, at Davao.

The so-called knee spread was a frequent method of torture. The victim, with his hands tied behind his back, was forced to kneel with a pole, sometimes as much as three inches in diameter, inserted behind both knee joints so as to spread those joints as pressure was applied to his thighs, at times by jumping on his thighs. the result of this torture was to separate the knee joints and so cause intense pain. THe evidence shows specific instances of this torture being used at the following places: China, at Shanghai and Nanking; Burma, at Tavoy; Andaman Islands, at Port Blair; Borneo, at Sandakan; Sumatra, at Pakan Baru; Moluccas Islands, at Halmahera Island; Portuguese Timor, at Dilli; Philippine Islands, at Manila, Nichols field and Pasay Camp; and in Japan, at Tokyo.

Suspension was another common form of torture. The body of the victim was suspended by the wrists, arms, legs or neck, and at time in such manner as to strangle the victim or pull joints from their sockets. This method was at times combined with flogging during suspension. Specific instances of the employment of this method of torture occurred in the following places: China, at Shanghai and Nanking; French Indo-China, at Hanoi; Malaya, at Singapore, Victoria Point, Ipoh and Kuala Lumpur; Thailand, at Chumporn; burma, at Kyaikto; Borneo, at Sandakan; Sumatra, at Brastagi; Java, at Bandung, Soerabaja and Buitenzorg; Moluccas Islands, at Amboina; Portuguese Timor, at Dilli; Philippine Islands, at Manila, Nichols field, Palo, Iloilo City and Dumaguete; and in Japan, at Tokyo and Yokkaichi.

Kneeling on sharp instruments was another form of torture. the edges of square blocks were mostly used as the sharp instruments, the victim was forced to kneel on these sharp edges for hours without relief; if he moved he was flogged. Specific instances of the use of this method have been shown to us to have occurred at the following places: French Indo-China, at Hanoi; Malaya, at Singapore; Andaman Islands, at Port Blair; Moluccas Islands, on Halmahera Island; Philippine Islands, at Davao; and in Japan, at Fukuoka and Omuta.

Removal of the nails of the fingers and toes also occurred. Instances of this method of torture are found at the following places: China, at Shanghai; Celebes, at Menado; Philippines, at Manila, Iloilo City; and in Japan, at Yamani.

Underground dungeons were used as torture chambers at the following places: French Indo-China, at Hanoi; Malaya, at Singapore; and in Java, at Bandung.

Flogging was the most common of the cruelties of the Japanese. It was commonly used at all prisoner of war and internee camps, prisons, Kempetai headquarters and at all work camps and on all work projects, as well as aboard prison ships. It was indulged in freely by the guards with the approval and often at the direction of the Camp Commandant or some other officer. Special instruments were issued for use in flogging at camps; some of these were billets of wood the size of a baseball bat. On occasions, prisoners were forced to beat their fellow prisoners under the supervision of the guards. Prisoners suffered internal injuries, broken bones, and lacerations from these beatings. In many instances, they were beaten into unconsciousness only to be revived in order to suffer a further beating. the evidence shows that on occasions prisoners were beaten to death.

Mental torture was commonly employed. An illustration of this form of torture is to be found in the treatment to which the Doolittle filers were subjected. After having been subjected to the various other forms of torture, they were taken one at a time and marched blindfolded a considerable distance. The victim could hear voices and marching feet, then the noise of a squad halting and lowering their rifles as if being formed to act as a firing squad. A Japanese officer then came up to 6the victim and said: "We are Knights of the Bushido of the Order of the Rising Sun; we do not execute at sundown; we execute at sunrise." The victim was then taken back to his cell and informed that unless he talked before sunrise, he would be executed.

Readers are welcome to read the rest of the document for other issues, like cannibalism