The family of a London, Ont. lifeguard is seeking answers after he fell off a cruise boat in Lake Ontario over the weekend.

Friends told CTV News that Keith White, 34, had flown to Toronto from London, Ont. with his girlfriend and another couple, and went on a lake cruise to "kick off the summer."

White and a group of friends from London were attending an event on the Northern Spirit I cruise boat on Saturday evening, when he fell off the boat into the water.

White's girlfriend told CTV Toronto he had leaned over the front of the boat and was "goofing around," saying that he wanted to go for a swim, when he toppled over the boat's railing.

"He just flipped. He lost his balance, and within a split second, he couldn't hold on anymore," Michelle Rodrigues said through tears on Monday.

Rodrigues said that both she and a security guard "bolted" for White, and that she tried in vain to hold onto his legs.

The incident happened in the Humber Bay area, approximately six kilometres west of the Toronto Islands, at approximately 7:40 p.m.

Witnesses said they saw him in the water, waving his hands and asking for help. A security guard immediately yelled man overboard, and Rodrigues raced to the back of the boat where she could still see White.

Rodrigues said saw him treading water and moving his head, but lost sight of him after 10 minutes.

"Then I blinked and he was gone."

The ship turned around, but by the time it got back to where White was last seen, he had disappeared.

Police estimated that the water was 9 C at the time of the incident. A person exposed to water at that temperature can experience hypothermia within 10 minutes.

In a statement sent to CTV Toronto by White's sister Tiffany, the family said he "embraced living life to the fullest," and loved adventures with friends and family.

"Keith lived life with a ferocity and love that was unparalleled," Tiffany White wrote.

"He was the person everyone loved being around, and the person that could make anyone laugh. He had a quick wit, a big heart and physical and mental strength that we all admired... He will be so deeply and truly missed, and we are trying to make sense of this senseless tragedy."

The White family said they are "confused" about the incident, saying that he was a certified lifeguard and healthy man.

They wrote that they didn't understand why it took so long for help to arrive, when he'd fallen off the ship during daylight hours and had been seen in the water alert and swimming.

"It appears nothing was done to assist him in a timely manner," the family wrote.

"We look forward to the investigation determining how his location in the water was lost so quickly in calm water in daylight."

The family statement said they're anticipating answers and, if required, a commitment for change in the industry.

Police said they are still treating the investigation as a missing persons case, hoping he will be found alive.

"He is still considered a missing person, (but) as time goes along the chances of a successful outcome are much slimmer," said Sgt. Gerry Klunder of Toronto Police's Marine Unit.

Rodrigues is also holding out hope that White may have survived because he is an experienced swimmer.

"I know he's somewhere," Rodrigues said.

However, water temperatures and the distance from the shore would have made a swim to safety a tough task, police say.

"A very, very strong swimmer would have a difficult time with that distance," said Klunder.

The Transportation Safety Board of Canada also sent a team to investigate on Monday.

The TSB says they will assess and gather information about the incident.

Officers have been using a small, sonar-equipped robot to search the bottom of Humber Bay. The Toronto Police Marine Unit is also searching the area approximately three kilometres south of the bay, in case the current from the Humber River moved White's body further into the lake.

Jim Nicholson, the head of Mariposa Cruise Lines, expressed his sympathy for White's family, and said staff members are co-operating fully with police.

Nicholson said his company hadn't experienced any similar incidents in its 28 years of operation.

With a report from CTV Toronto's Colin D'Mello and files from The Canadian Press