I've been researching this question and I can't really find a definite anwser... Most communities also asked this kind of question and the answers are different. Some people may have got lucky and catched Lugia with a dive ball, then say that dive ball is effective on Lugia. Bulbapedia says that Dive Ball is effective on Pokemon that are found by surfing, fishing, and in the seaweed. But Lugia is a still legendary Pokemon that always stays there before you battle it, it may be different from other wild Pokemon that are encountered by fishing surfing and seaweed. I don't think that this question can be anwsered unless there is a reliable source from Bulbapedia or Serebii.