It was a long holiday weekend in Colombia and the El Almirante ferry was filled with tourists.
Soon after the boat began cruising around the reservoir in Guatape on Sunday, survivors described hearing a loud explosion near the men's bathroom which knocked out power.
Shortly after that, the boat began sinking. The first two floors went underwater quickly as people rushed to get up to the fourth floor.
Colombia's President Juan Manuel Santos went to the area where the boat went down and said at least six people were dead and another 31 were missing. He said another 122 people, of the 160 on board, were either rescued or were able to make it to the shore.
The Associated Press reports:
"It's unclear what caused the boat to sink.
"Some people who witnessed the tragedy from the nearby shore said the boat appeared to be overloaded but Santos said it was sailing well below capacity. None of the passengers were wearing a life vest. Complicating the search, there wasn't even a passenger list.
"Nobody really knows what happened," said Santos, adding that naval officials were brought in to carry out an investigation.
"Carlos Espinosa, an independent journalist from Guatape, said about a month ago townspeople awoke to find the El Almirante filled with water and sinking at its dock, suggesting that perhaps the vessel wasn't ready to return to the water."
An even deadlier incident was avoided, as recreational boats and jet skis rushed to the scene to pull people from the party boat as it went down.
Relatives and friends urge rescue workers not to stop searching for those missing.