Along with this now familiar general warning, the FBI has introduced the specter of a new terrorism threat: booby-trapped beer coolers. A lightly classified bulletin sent to 18,000 state and local agencies last week advised local authorities to look out for plastic-foam containers, inner tubes and other waterborne flotsam commonly seen around marinas that could be rigged to blow up on contact. Also, the bulletin warned, terrorists might attach bombs to buoys. FBI and Department of Homeland Security officials say no such devices have actually been discovered, nor is there any current intelligence that terrorists are hatching plots involving floating bombs. But some officials believe al-Qaeda may be focusing on harbors and shipping channels in an effort to replicate the success of the October 2000 attack on the U.S.S. Cole in the port of Aden, in which suicide bombers used a small launch laden with explosives to rip a 40-ft. hole in the warship's hull, killing 17 sailors.