The deaths of five British nationals on a Canadian whale-watching tour is a rare black eye for a multi-billion-dollar industry that experts say generally operates safely, giving millions of tourists annually an up-close view of the world’s largest animals.

Five British nationals died Sunday when the Leviathan II capsized off the coast of Vancouver Island, according to the U.K. government. But such incidents are rare in an industry with more than 3,000 operators around the globe, say experts.

“It’s very, very unusual,” said Capt. James Staples, a longtime ship’s master, instructor and maritime safety consultant. He said whale-watching passengers face greater risks tripping or slipping on wet decks or staircases than they are for drowning at sea. “It’s a very safe industry and very seldom do you ever hear of a major incident on any type of whale-watch vessel.”

Whale-watching boats usually race toward the area where the massive animals are swimming. The boats then idle in place as passengers rush from side to side to snap photos and videos. Tour operators generally halt their propellers to remain at a distance from the whales, although the massive mammals then often swim up to the boats.

The movement of the passengers likely wouldn’t have been enough to flip a boat of that size, said Staples.

Whale watching is a $2.1 billion-and-growing industry, serving more than 13,000 tourists in 119 countries, according to a 2009 report by the International Fund For Animal Welfare. North America is the single-largest market for whale watching, with boats certified by either the U.S. or Canadian coast guards. Staples said coast guard regulations require licensed captains and crew, along with safety equipment such as life jackets.

The boats operate close to the coast or miles offshore, depending on where the whales are at any time. Where the whales are swimming typically dictates the size of the boats used. The Leviathan II, which was operating near the Vancouver Island Coast, is 65 feet long and has a single hull. Whale-watching boats operating further from shore often are much larger and have twin hulls, which makes them more stable in the waves.

Given the generally calm weather conditions off Vancouver Island and the number of passengers aboard, Staples suspects a hull breach swamped the Leviathan II. “Something allowed the hull to fill with water, because the weather wasn’t that bad,” he said.

Added Capt. Joseph Murphy II, a professor of marine transportation at the Massachusetts Maritime Academy: “This has to have been some form of sudden incident. My suspicion is that they got water in the engine room.”

The company says the Leviathan II is crewed by Canadian-licensed sailors and certified by Transport Canada.

Staples said a boat this size would likely not have had a dedicated engineer monitoring the engine below-decks, but monitors should have alerted the captain if the ship’s lower levels began filling.

“Our entire team is heartbroken over this incident and our hearts go out to the families, friends and loved ones of everyone involved,” Leviathan II owner Jamie Bray said in a statement.

Staples said certification by Transport Canada likely means the boat – unless improperly modified – would have been designed to handle shifts in weight by passengers. The April 2014 capsize of the South Korean ferry Sewol has been attributed to illegal modifications that made the ship heavier and capable of carrying more passengers. The ferry capsized in part because it was carrying too much cargo and not enough ballast, material that would have helped keep the ship stable in the water as it turned. Staples said without enough ballast, the ferry began to list, and then unsecured cargo slid inside the hold, exacerbating the problem.

“I wouldn’t think that was the cause here,” he said of the Leviathan II. “With the shift of weight, it can happen, but it’s very difficult for that to happen unless there’s been a major change to the vessel’s structure.”

Murphy said it’s likely that once the whate-watching boat began filling with water, panicked passengers may have rushed to retrieve life jackets, which may have been stored on racks inside the main cabin. Depending on how fast the boat was capsizing, those passengers may have gotten trapped inside as the cabin filled with frigid ocean water, he said. Staples said boats like this rarely carry or require passengers to wear special survival suits allowing them to withstand the cold water. An automated weather station near where the incident happened says the water temperature was about 55 degrees at the time.

“The water’s cold and people don’t have a lot of time to survive – it would be just a few minutes and they might have been trapped inside,” Murphy said.