A duck boat carrying 32 passengers overturns on the Charles River near a Cambridge boat ramp, injuring six people, two of them seriously, the Cambridge Police Department says.Boston Duck Tours suspends operations for the day and is cooperating with the Massachusetts State Police and the US Coast Guard as authorities lead an investigation into why the amphibious vehicle flipped.Witnesses describe hearing a crash and seeing first responders at the Education Street ramp, responders account for all aboard, and locals say the accident is prompting renewed scrutiny of duck-boat safety.