FOUR people who drowned an alleged triad boss in a basin of water were found guilty of manslaughter by a High Court jury yesterday. They were also convicted of forcible detention. The jury, however, returned a ''not guilty'' verdict on the three men and a woman accused of murdering Tong Yim-tsan, 41. Printing artist Leung Siu-hong, 23, his 24-year-old lover, Wong Pui-ling, and cleaning worker Chan Chi-ho, 26, were found guilty of forcibly detaining Tong for a $2 million ransom. Lee Kwok-wah, 18, was found guilty of false imprisonment. Justice Stock deferred sentencing to May 7. The court heard during the trial the alleged ''big brother'' owned part of a Wan Chai karaoke lounge where Leung and his lover were employed but they resigned after Leung had a row with Tong over Wong. Crown counsel Mr Stevan Cole said the four waylaid Tong in the carpark of his Tsuen Wan home on December 20, 1991, and took him to Leung and Wong's Yuen Long flat. Tong was allowed to contact his wife the next day, telling her to deposit between $80,000 and $100,000 into his bank account. Leung however demanded more money from Mrs Tong after killing her husband because he thought Tong was tricking him when he said he had only $300,000 in his account. Tong was dragged to the kitchen where the three held Tong while Leung pushed his head into a basin of water, drowning him. According to Wong's statement after their arrest on December 31, 1991, they killed Tong because they feared he would retaliate.