TWO teenage boys who flew out of Hong Kong yesterday had every reason to be apprehensive as they boarded a flight for Canada. But if they showed any ''fear of flying'', it had little to do with catching a plane. Wat Tze-wan, 17, and Yu Wai-cheung, 19, have a date with the world's best judo players at the world championships, which start on Thursday in Hamilton - and are likely to be flying across the mat if they come up against any of the top-ranked players. The championships represent a giant leap for these two players, part of a seven-strong Hong Kong team being accompanied by national coach David Starbrook. Their only previous taste of international competition has been an Interport match against Macau andGuangzhou. ''It can be a good experience for them. They will see just what top-level judo is like,'' says Starbrook, a two-time Olympic medal winner for Britain. But Hong Kong's chances of winning even a single contest can be gauged by their performance at last year's Barcelona Olympics. They sent four very experienced players, all of whom lost at the first hurdle - three of them in less than a minute. While Wat (60 kilograms) and (65 kgs) Yu lack experience, the same cannot be said of Hong Kong's third men's representative at Hamilton - 35-year-old Chong Siao-chin (71 kgs), who won bronze at the 1986 Asian Games. Chong has not represented the national team for three years but earned his recall at July's Hong Kong Open - which doubled as a selection trial. The ''babe'' of the four-strong women's contingent is Cheng Wah-fong (48 kgs), although at 25 she is their second-oldest representative. Her first chance at top-level international competition comes with the decision by the territory's long-time 48 kgs number one Chan Mei-ling to step up to 52 kgs. Cat Law Lai-wah (61 kgs) and Mo Ching-wai (65 kgs) make up the women's team.