Bangkok  - Marine police launched a sea surface search Tuesday near the Similan Islands for six missing foreign tourists and a Thai national whose scuba diving boat sank after being hit by a sudden storm.

"We are conducting a surface search in the area today," Marine Police Lieutenant Colonel Wanlop Phuangbaka said. "If the bodies are not in the boat they should float to the surface within 24 hours," he said in a telephone interview with Deutsche Presse-Agentur dpa.

The missing foreigners include three Austrians, two Swiss, a Japanese and a Thai man, said Beno Branden, a manager at the Dive Asia Company which runs scuba diving tours to the Similan Islands.

The Choke Somboon, the company's 29-metre scuba-diving-boat-cum-floating-hotel, sank late Sunday night in the Andaman Sea after being hit by a sudden tornado, according to crew and survivors.

Thai authorities are using a helicopter to search the area where the boat sank.

"The whole affair took one to two minutes at the most," Branden said. "The captain said it was a twister. It was 2-3 kilometres away and then at the boat within a minute."

Some 23 passengers, including 15 foreigners and eight Thai staff, made it to life rafts and were rescued by a Thai fishing boat Monday morning.

Most of the passengers were in the boat's saloon deck when the twister hit at about 11 pm Sunday (1600 GMT), about 30 kilometres offshore of Phuket Island, a popular Thai beach resort situated about 600 kilometres south of Bangkok.

It was unclear whether the missing passengers were inside the boat when it sank.

"We have relatively certain information that at least two of the missing people made it out of the boat because some survivors saw them and described them as going in the opposite direction," Branden said.

But the likelihood of them surviving at sea was deemed slim.

"Everything happened so quickly I don't think the people had time to take life jackets," Branden said.

Those picked up from the sea were reported to be well, with only sunburn and scratches as injuries.

The boat itself is advertised on the company's website as it's "luxurious flagship," with air-conditioning, a saloon, a bar, a multi-media centre and a large sun deck.

A typical tour takes scuba divers to the Similan Islands from Phuket, allowing them to dive all day and then sleep on the boat as it drives back to Phuket overnight, which takes 10 to 12 hours.

The Similan Islands, a national marine park, is one of Thailand's best-preserved scuba and snorkeling destinations. Hotels and guest houses are prohibited on the islands to conserve the environment. (dpa)