I think it could have all started from misplaced causality. Once upon a time, someone smoking a big-ass cigar while walking along the beach came upon a group of people around someone who had drowned. Considering that at the time people thought cigar smoke was actually healthy, it wouldn't be difficult to imagine they blew smoke into everything except the mouth, as water was coming out. When the air pressure was applied to the anus, depending upon how lustily the smoke was blown in, could displace space in the abdomen, pushing water out of the lungs. Then everyone would only need attribute the benefit to the smoke, not the application of internal pressure. (Queue survivor fart jokes.)