“Those on the first and second floors sank immediately,” a female survivor who was not identified told Teleantioquia, a public television network in Colombia. “The boat was sinking and all we could do was scream and call for help.”

Margarita Moncada, the head of the disaster response agency in Antioquia state, said that according to a preliminary report 99 people had been rescued and 40 others had made it to shore safely and in good condition.

The authorities put out a call for scuba divers to help with the search for the missing.

Daniel Giraldo, owner of an Italian restaurant in Guatape, said he went to the reservoir after hearing the sound of ambulances. When he arrived, people told him the ship had gone under. “It sank in a matter of four minutes,” he said.

Mr. Giraldo said he then he went to the hospital where he saw a baby girl in a wet dress who had been saved but whose mother, he was told, was missing.

The authorities could not say exactly how many people were on the boat and asked passengers or relatives to report to a makeshift rescue center along the shore of the reservoir, which surrounds the soaring rocky outcrop of El Penol, a popular weekend destination a little more than an hour from Medellin. It was especially crowded on Sunday as Colombians celebrated a long holiday weekend.