Scuba divers have continued searching for bodies in a reservoir near the Colombian city of Medellin where a tourist boat packed with more than 150 passengers capsized, leaving at least seven people dead and 13 missing.

After suspending their search overnight due to a lightning storm, scuba divers by mid-morning yesterday had pulled a seventh body from the underwater wreckage.

Authorities were also turning their attention to the causes of the sinking and whether the company that owned the boat named El Almirante contributed to the accident.

A flotilla of recreational boats and jet skis had rushed to the scene, pulling people from the boat as it went down and avoiding an even deadlier tragedy.

Dramatic videos circulating on social media show the turquoise-and-yellow-trimmed party boat rocking back and forth as people crawled down from a fourth-deck roof as it sank in a matter of minutes.

Survivors described hearing a loud explosion near the men's bathroom that knocked out power a few minutes after the boat began its cruise around the giant lake.

As water flooded on board, pressure built and people were sucked under by the sinking ship.

"Those on the first and second decks sank immediately," survivor Lorena Salazar told local media.

"All we could do was scream and call for help ... it was completely chaotic."

In the absence of a passenger list, authorities have been relying on family numbers and survivors to report their whereabouts.

This morning, the number of people missing was down to 13.

Of those who survived the incident, three remain in hospital but are out of danger, said Margarita Moncada, the head of the disaster relief agency in Antioquia state.

A group of a few dozen scuba divers are working in hour-long shifts, looking to sweep for trapped bodies in the frigid, algae-filled waters around the wreckage at a depth of more than 30 metres.

Moncada said the hardest part for scuba divers is to safely search the area around the first deck of the boat.

It is unclear what caused the boat to sink.

Some survivors and people who witnessed the tragedy unfold from the nearby shore said the boat appeared to be overloaded.

But President Juan Manuel Santos, who travelled to Guatape to oversee search efforts, said it was sailing well below capacity.

None of the passengers was wearing a life vest.

"Nobody really knows what happened," said Mr Santos, adding that naval officials were brought in to carry out an investigation.

The reservoir surrounding the soaring rocky outcrop of El Penol is a popular weekend destination a little more than an hour from Medellin.

It was especially busy on Sunday as Colombians celebrated a long holiday weekend, some of them taking a five dollar pleasure cruise on El Almirante.

The transportation ministry said the company that owned the boat, and 12 others, had its certification renewed in December.

Carlos Espinosa, an independent journalist from Guatape, said a few months ago local people awoke to find the El Almirante filled with water and sinking at its dock, suggesting that perhaps the vessel was not ready to return to the water.

"What makes you angry is there are no controls by the government," he said.

As night fell, the usually festive town was silent as people began to register the magnitude of the loss.

Among those huddled under the rain near the port looking for information about loved ones was Alberto Villegas, who was separated from a cousin and uncle in the mad rush to abandon the sinking ship.

"All we ask is that they don't give up the search," said Mr Villegas.