The man who died yesterday morning after his motorboat collided with a dinner-cruise ship on the Potomac River near the 14th Street bridge has been identified as John C. McGinnis, 30, an Arlington County police officer.

McGinnis, of Falls Church, was alone in his 26-foot cabin cruiser at 9:14 p.m. Wednesday when it collided with the 97-foot cruise ship Dandy just north of the bridge, said Petty Officer Mike Waller, a spokesman for the Coast Guard's marine safety office, which is investigating the accident.

McGinnis, who was off duty at the time of the accident, was removed from the water by officers of the D.C. police's harbor branch and was rushed by helicopter to the Washington Hospital Center. He was declared dead at 12:15 a.m. yesterday.

No other injuries were reported.

The Coast Guard is investigating the accident but has not determined the cause, Waller said. D.C. police officials said, however, that a preliminary investigation shows that McGinnis was at fault in the accident.

Both craft were damaged in the bow area, although damage to McGinnis's boat was more severe, Waller said.

McGinnis, who was assigned to the police department's uniform patrol division, was unmarried and had no children, Arlington police said.