Drug Trafficking Vessel Interdiction Act of 2008

Drug Trafficking Vessel Interdiction Act of 2008 - Amends the federal criminal code to impose a fine and/or prison term of up to 15 years for knowingly operating or embarking in any submersible or semisubmersible vessel that is without nationality and that is navigating in, or has navigated into, through, or from, waters beyond the outer limit of the territorial sea of a single country or a lateral limit of that country's territorial sea with an adjacent country, with the intent to evade detection. Grants extraterritorial federal jurisdiction over violations of this Act.

Makes it an affirmative defense to a prosecution under this Act that a vessel operated at the time of a violation was: (1) a vessel of the Untied States or lawfully registered in a foreign nation; (2) classed by and designated in accordance with the rules of a classification society; (3) lawfully operated in a government-regulated or licensed activity; or (4) equipped with and using an operable automatic identification system, vessel monitoring system, or a long range identification and tracking system. Specifies the documents required to conclusively prove an affirmative defense.

Directs the U.S. Sentencing Commission to promulgate or amend existing guidelines to provide adequate penalties for violations of this Act.