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 attempting or conspiring to operate by any means, or for embarking in, any submersible or semi-submersible vessel that is without nationality and that is navigating 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 avoid detection. Imposes an additional civil fine of up to $1 million for violations.