They are illegal. I refer you to first three articles of the UN Convention against Torture, to which we are a party nation (signed in 1988, ratified in 1994):
Article 1
1. For the purposes of this Convention, 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. 2. This article is without prejudice to any international instrument or national legislation which does or may contain provisions of wider application.
Article 2
1. Each State Party shall take effective legislative, administrative, judicial or other measures to prevent acts of torture in any territory under its jurisdiction.
2. No exceptional circumstances whatsoever, whether a state of war or a threat or war, internal political instability or any other public emergency, may be invoked as a justification of torture.
3. An order from a superior officer or a public authority may not be invoked as a justification of torture.
Article 3
1. No State Party shall expel, return ("refouler") or extradite a person to another State where there are substantial grounds for believing that he would be in danger of being subjected to torture. 2. For the purpose of determining whether there are such grounds, the competent authorities shall take into account all relevant considerations including, where applicable, the existence in the State concerned of a consistent pattern of gross, flagrant or mass violations of human rights.
I then refer you to the Senate reservations to said document, which further clarified what constitutes "mental torture":
II. The Senate's advice and consent is subject to the following understandings, which shall apply to the obligations of the United States under this Convention:
(1) (a) That with reference to article 1, the United States understands that, in order to constitute torture, an act must be
specifically intended to inflict severe physical or mental pain or suffering and that mental pain or suffering refers to prolonged mental harm caused by or resulting from (1) the intentional infliction or threatened infliction of severe physical pain or suffering; (2) the administration or application, or threatened administration or application, of mind altering substances or other procedures calculated to disrupt profoundly the senses or the personality;
(3) the threat of imminent death; or (4) the threat that another person will imminently be subjected to death, severe physical pain or suffering, or the administration or application of mind altering substances or other procedures calculated to disrupt profoundly the senses or personality.
I then refer you to Former Acting Attorney General Daniel Levin's memo regarding waterboarding (which he had firsthand experience with, having subjected himself to the procedure as part of his investigation of whether or not it was torture), which confirms that waterboarding as practiced by the U.S. is, in fact, illegal under the terms of the UNCAT. I also refer you to the firsthand testimony of people who have been waterboarded as part of special ops training, who insist that even knowing that the procedure would be interrupted if they showed serious discomfort or if they were in danger they still could not shake the certainty that they were going to die. Thus, waterboarding is a form of mock execution--which is made explicitly illegal by the Senate reservations to the UNCAT.
Waterboarding is illegal torture under both U.S. and international law. There is no bordering about it--and anyone with a computer and internet access can confirm it for him or herself, the relevant documents are all online.
The UN Convention against Torture:
http://www2.ohchr.org/english/law/cat.htmThe Senate Reservations to the UNCAT:
http://www2.ohchr.org/english/bodies/ratification/9.htm#reservationsDaniel Levin's memo:
http://www.usdoj.gov/olc/18usc23402340a2.htm