Could the ratio of visitor deaths to locals simply be a matter of experience combined with frequency. When a visitor comes to Hawaii for a week, there is a probably good chance they will snorkel once or more during that week. Where as, most locals do not snorkel once per week or more. Put another way, if you did a moment in time snapshot population of snorkelers, wouldn't you expect to find a ratio of 9 visitors to 1 local in the water at any given moment?