The airplane was departing on a wet runway. About halfway down the runway, during rotation, the left nose cargo door opened. The pilot aborted the takeoff, but the airplane traveled off the end of the runway and struck an embankment. Review of the airplane information manual revealed that an open cargo door would not strike the propeller or cause any unusual handling characteristics; and that a takeoff with an open cargo door could be successfully completed. Examination of the airplane did not reveal any pre-impact mechanical discrepancies, nor did the pilot report any.