Police have pressed charges against six people in connection with the drowning of actress Nida "Tangmo" Patcharaveerapong.

Detectives determined the actress fell from the stern of a speedboat and was hit by its propeller before drowning in the Chao Phraya River late in February.

Police said Tanupat "Por" Lerttaweewit (the speedboat owner) was charged with recklessness causing death, driving a boat without a licence, dropping objects into the river, giving false statements, failing to attach the boat name onto a vessel and driving a boat with an expired registration.

Phaiboon "Robert" Trikanjananun was charged with recklessness causing death, driving a boat without a licence, using a boat with an expired registration and dropping objects into the river.

Wisapat "Sand" Manomairat faced charges of recklessness causing death.

Nitas "Job" Kiratisoothisathorn was charged with concealing evidence to help offenders and dropping objects into the river.

Idsarin "Gatick" Juthasuksawat (Tangmo's manager) was charged with concealing evidence to help offenders and giving false statements.

Peam "Em" Thamtheerasri was charged with concealing evidence to help offenders and having others make false statements.

The first five suspects were on the speedboat with Tangmo on the Chao Phraya River on the night of Feb 24. The actress fell from the boat and drowned.

Police said surveillance camera footage showed an object at the stern of the boat near Pibun 1 pier in Nonthaburi at 22.33pm on Feb 24. One minute later, the object had disappeared.

At the next moment, a person stood up on the boat, which then slowed down and circled in the river.

Police found that Tangmo's body sustained 26 wounds between her thigh and her ankle.

The biggest wound, on the inner side of her right thigh, was 7 centimetres wide, 26cm long and 1.5-4.5cm deep. It matched the dimensions of the speedboat's propeller blade.

"Tangmo did not [just fall from the boat], there was a reckless act that caused her death," a police commisioner said.

Police found no trace of any assault on Tangmo's body,

"When (Tangmo) was hit by a propeller, she would have cried out. Water would have filled her lungs and she would have drowned immediately. The big cut in her thigh would have prevented her from moving her leg so she could not swim," he said.

He also said everyone aboard the boat was drunk, and someone told the five people who remained on the boat to delay meeting police to avoid being prosecuted for driving under the influence of alcohol. They were advised to tell police they had drunk only a little.