SINGAPORE — When Indian architect Rahul Maini and his parents embarked on their first trip abroad in May, Singapore was their destination of choice.

But the trio wasn’t going for the hawker food or even the city-state’s casinos — they were there to get on a ship.

The equatorial island has become a flourishing entry point for Indian cruise-ship passengers, bolstering sales for operators, including Royal Caribbean Cruises Ltd. and Genting Hong Kong Ltd.

About 100,000 Indians sailed from Singapore last year, 29% more than in 2015, making India the biggest market for cruises departing from the Southeast Asian nation, according to the Singapore Tourism Board.

“We chose to go on a cruise because we could visit three countries in one short trip,” said Mr. Maini, 26, whose four-day cruise on Royal Caribbean’s Voyager of the Seas took in the Malaysian city of Penang and the Thai island of Phuket.

The family spent about $7,700, which Mr. Maini said was “expensive, but worth it.”

The expenditure is part of the 777.3 billion rupees ($12 billion) that Euromonitor International predicts middle-class Indians will shell out on overseas leisure travel this year. The market is expanding about 10% annually and will eclipse 1 trillion rupees by 2020, the research company says.

While the Middle East and France are the most-popular overseas destinations for Indians, Singapore is expected to register a 59% jump in arrivals from the world’s second most-populous country from 2015 to 2020, according to Euromonitor. Among the city-state’s top 10 inbound passenger markets, India is the fastest-growing, according to Changi Airport Group, which manages Singapore’s international airport.

The number of arrivals from India increased 15% in the first five months of this year, compared with a year earlier — outperforming China by three percentage points. Many of the tourists are like the Mainis, who come mainly to join a cruise.

“Fly-cruise tourism has really taken off among Indian tourists,” said Chayadi Karim, a research associate with Euromonitor.

To help, the Singapore government created the Cruise Development Fund, which supports travel agents and event organizers trying to get people to sail from Singapore, said Annie Chang, a director at the tourism board.

The number of Indian passengers on Royal Caribbean ships has jumped 149% so far from a year ago. This includes the peak summer school holiday period that runs in India from May to June, said Sean Treacy, the company’s Asia Pacific managing director.

“Singapore is a regional-hub port which is near many attractive Southeast Asian cruise destinations in Malaysia, Thailand and Vietnam,” Mr. Treacy said.

Voyages from Singapore offer Indian tourists the convenience of visiting multiple destinations across different countries on a single trip while unpacking only once, he said.

The number of cruise passengers from India leaving via Singapore has been increasing by least 10% a year annually, said Michael Goh, senior vice-president of international sales for Genting Cruise Lines, and the company is “optimistic” about continued growth.

“Perceptions of cruising among Indian travelers are fast-changing,” Mr. Goh said, adding that the unit of Genting Hong Kong counts among its cruising clientele first-timers, singles, couples, senior citizens, multigeneration families and Indians with a higher disposable income.

Princess Cruises, a unit of Carnival Corp., also sees “positive growth from the Indian market,” including honeymooners, said Farriek Tawfik, the company’s Southeast Asia director.

There’s also a tendency among Indian expatriates living in Singapore to invite relatives to join them on a cruise, he said.

Royal Caribbean is adding more cruises for India’s summer school holidays, Mr. Treacy said.

Voyager of the Seas will go on 18 cruises during the holidays next year, 14 more than in 2016. The number of cruises for Mariner of the Seas will almost double to 56 during the upcoming winter-spring season from 29 in 2014-2015, he said.

To better accommodate guests from India, the cruise operators offer vegetarian meals, local cuisine and culture, and special events that appeal to South Asian guests.

“More Bollywood music may be played at the pool or disco parties, and more jewelry gift sets, which are popular with Indians, may be procured for sailings that host a higher number of them on board,” Royal Caribbean’s Mr. Treacy said.

As for Mr. Maini, he said his inaugural overseas holiday has given him the travel bug.

“Singapore was good, but the cruise was better,” the New Delhi resident said.

He’s now saving for a cruise from Barcelona, he said. — Bloomberg