Search teams in Canada have launched a recovery effort for six people believed to have drowned in a “bizarre” sinking of a fishing charter off the coast of Vancouver.

Police and rescue crews praised a couple who were passing in their yacht for making a critical mayday call and saving stranded passengers by pulling them onboard their craft.

The Royal Canadian Mounted Police (RCMP) said its underwater team was preparing to search for the vessel. It is believed to have been carrying 10 people before it took on water and disappeared into the depths of the Georgia strait close to where fresh river water meets the ocean, creating hazardous survival conditions.

Two survivors, a man, 33, and a woman, 28, were in a critical condition. Another man, 26, and woman, 33, were discharged from hospital.

Little is known of what unfolded on the boat, which departed from the community of Steveston and sank around midday on Sunday. Officials said none of the passengers were wearing lifejackets and there was no mayday call from the charter.

Stephen Adam, an operations manager with Royal Canadian Marine Search and Rescue, said the incident was “bizarre” because the boat had sunk so quickly and had not issued a distress call. He said he did not “have any details of the type of vessel it was, why it went out, where it came from”.

When rescue teams arrived the boat had sunk. Given the speed with which it disappeared and the lack of a distress call, the RCMP’s major crimes unit is leading the investigation to determine if there was a collision or any criminal behaviour.

Maj Gregory Clarke of the Joint Rescue Coordination Centre said people could survive for as long as 10 hours when wearing a flotation device, but the “pretty unforgivable” cold waters and strong currents meant survival rates were “cut very short”. He added: “[There] was no indication of any distress when whatever happened, happened.”

Crews scanned the suspected area for hours. One air force plane flew in a grid pattern over the search area for about seven hours.

Dorothy Stauffer and Brian Angus were sailing in the area on Sunday when they spotted people in the water. Stauffer, who received emergency training while working as a flight attendant, told CBC News the group had appeared weak and hypothermic.

One had no clothes on from the waist up, Stauffer said. The people in the water seemed confused as she tried to coax them to swim to the dinghy and grab on to its side or tow rope. It took some survivors nearly 20 minutes to make their way to the small boat.

Stauffer and Angus initially saw five people in the water, but quickly lost sight of one who disappeared beneath the surface. The couple were able to rescue three people and search teams retrieved a fourth.

“We lost sight of the other two, we decided to just go for the three that were closer together, that’s the decision – a hard one – we had to make,” Angus, a retired pilot, told CBC.

“The question you have in any incident as a pilot … or a boater is: could we have done anything different? And we don’t believe we could have.”