POLICE raided a floating casino in Shau Kei Wan typhoon shelter yesterday, arresting 38 people and seizing betting chips with a face value of $2 million. Five officers posed as customers in the operation, said Inspector Benson Ma Siu-wai. ''We'd been watching this particular casino for about three weeks,'' said inspector Ma. It is believed the casino, operated by a triad gang, had a daily turnover of about $1 million. ''To evade police detection, another vessel was moored in front to provide cover,'' said Inspector Ma. ''In addition, the connecting sampans usually switched off their lights when they sailed close to the casino. Customers had to mention several nicknames before they were allowed to gamble.'' Of those arrested two men, aged 23 and 32, are believed to be the operator and the assistant operator of the casino. They were released on bail of $20,000 and $5,000 respectively. The remainder are alleged gamblers, aged from 20 to 46, who had all been allowed bail of $500 each.