murder.....(this is all hypothetical...don't send the police...or the people in the white jackets..... )
Obtain Gamma Hydroxybutyrate (GHB). Not screened for in routine toxicology. Leaves the system quickly. In large doses can cause coma, respiratory depression and unconsciousness.
Slip someone the GHB in a glass of alcohol. Wait for effects. Take them to a pool/lake/body of water. Drop them in.
Cause of death: accident by drowning.
Kids: Don't try this at home.