Leonardo DiCaprio was forced to swim for his life off the coast of Thailand when the boat he was in started to sink in a monsoon downpour.

In an apparent case of life imitating art, the Titanic star had to leap into the sea last Friday after winds whipped up five-metre-high waves, flooding the boat and dragging it out to sea.

DiCaprio and co-star Tilda Swinton were riding in a long tail boat - a traditional fishing canoe with an outboard motor - when it went down Friday in heavy seas during a sudden storm. The two were filming The Beach off Phi Phi Island in the Andaman Sea.

DiCaprio, 24, and Swinton, 31, and crew members spent half an hour swimming at sea before rescuers arrived. They had all set off in two boats, and the storm hadn't started until they were a mile offshore.

Thai newspapers speculated that the film crew's disrespect of local guardian spirits had caused the sinkings. Twenty years ago, two young lovers made a suicide pact in the area, in the very sort of story DiCaprio would star in if it were a Hollywood film. "Now it is said that the film-makers firmly believe in the guardian spirits and vowed that they will not make the same mistake twice," a Thai web site devoted to following the filming reported. The Thai Daily News added that the film crew was planning to make an offering to the spirits immediately.

Contrary to reports in Thai tabloids and in the Sun, no sharks were seen near the boat, said Santa Pestanij, the Thai coordinator for the 20th Century Fox film who was present when the boat capsized.

Santa said the boat accident cost tens of thousands in damage to camera equipment.

"We did not underestimate the sea. There were at least four security boats present and prepared for the unpredictable," Santa said.

Dive operators contacted by The Associated Press said some small sand sharks might swim near the islands, but they had never heard of a shark attack in the area.

Santa said Dicaprio remained calm for the five to ten minutes he was in the water and helped soothe crew members who feared drowning.

"He was a real gentleman. A hero," Santa said.

DiCaprio is anything but a hero, however, to local environmental activists who are suing him and the film company in a Thai court for allegedly damaging the ecosystem of Maya Beach on the island.

The activists have staged several protests against DiCaprio, wearing masks made from a photo of his face with the addition of vampire fangs. Security has been extremely tight on the set to keep the protesters away.

The film is scheduled to finish shooting on Saturday, but Santa said DiCaprio wants to spend a few more days in Thailand and make a trip to Bangkok.