Cut cucumber lengthwise in quarters. Slice each quarter into 1 cm slices. Peel pineapple and cut into quarters. Remove core. Cut each quarter in half and slice into 1 cm slices.
Cut red onion in half and thinly slice each half. Remove thick stalks from watercress and reserve leaves. Combine cucumber, pineapple, onion and watercress leaves in a bowl. In another bowl, combine lemon juice, soy sauce, sugar and chili sauce. Pour over salad and toss together. Serve on a platter.