“Cruise products are very popular with seniors, families and honeymooners in China,” she said.

On board one of the world’s largest cruise ships

About 30 million people worldwide are expected to go on a cruise this year, up nearly 70 per cent from a decade ago, according to the Cruise Lines International Association.

While the more mature US market remains the largest globally, China is growing fast – last year about 2.4 million Chinese tourists went on a cruise, more than triple the number in 2014.

Gavin Smith, senior vice-president international of US cruise giant Royal Caribbean said there was rising demand right across Asia with growth at double digits in some countries.

“The Asian market is increasingly important to global cruising,” he said.

Modern liners resemble floating, futuristic cities capable of carrying thousands of passengers, where robot bartenders serve drinks and passengers can enjoy hi-tech entertainment.

But the rising number of monster liners, often hundreds of metres long and several stories high, has increased concerns about environmental damage.

“We see a big range of environmental affects from the cruise industry – everything from air pollution to waste water, sewage, oily discharges, food wastes, plastics,” Marcie Keever, oceans and vessels programme director at Friends of the Earth US, said.

A major concern is high levels of sulphur oxide emissions, a toxic gas which causes respiratory problems and lung disease, and can lead to acid rain and damage aquatic species.

In 2017 ships operated by Carnival Corporation – the world’s largest cruise company, which has several brands – emitted 10 times more sulphur oxide in European seas than all the passenger vehicles in the continent, according to a study by European NGO Transport & Environment.

Efforts are being made to reduce the amount of the gas emitted. New industry standards will from next year require all ships to cut sulphur oxide content in fuel to a maximum 0.5 per cent from the current 3.5 per cent limit.

Cruise ship slams into Venice wharf sending tourists fleeing

There are also worries about the impact of mammoth vessels on the cities where they berth during voyages.

A 13-deck liner crashed into a tourist boat in a Venice canal last month, injuring four tourists, and prompting a protest from local residents who called for a ban on large cruise ships in the Unesco World Heritage Site.

In addition, ships jettisoning waste at sea and the huge amounts of electricity used by the vessels have angered environmentalists. In June Carnival was fined US$20 million in the US for dumping plastic waste into the ocean and other environmental violations.

Industry players insist they are doing their part to reduce pollution.

Royal Caribbean has installed systems that clean almost all sulphur oxide and other pollutants from emissions, according to Nick Rose, the firm’s director for environmental programmes.

“Cruise ships are cleaner than ever before and improving with each new class of ship,” he said.

Royal Caribbean’s Spectrum of the Seas – billed as the region’s biggest cruise ship – is typical of the new generation of liners. The 345-metre (1,140 feet) vessel can carry more than 5,600 guests. It was custom-built for the Asian market, catering to Chinese tourists who often go on cruises in large groups.

There are 17 restaurants on board and the huge range of accommodation includes a two-storey family suite that comes with a private karaoke room and children’s slide.

Entertainment includes simulated skydiving in a wind tunnel, bouncing on a trampoline while wearing a virtual reality headset, and bumper cars, all aimed at an increasingly younger generation of cruise-goers.

Passenger Sylvia Bau, a veteran cruise-goer on holiday with six relatives, was amazed at the rapid growth of liners over the years.

“You can live in the ship for days without getting bored,” the 58-year-old Singaporean said as the vessel made a call in the city state, one of several stops in Southeast Asia before arriving at its home port of Shanghai.