Sorry to hear about this, glad to know your group feels better now. Hundreds if not thousands of visitors snorkel everyday. In all the years this board has been in operation (since 1997) nothing like this has ever been reported from snorkeling.
For a single day illness to happen two years in a row to an entire group seems to be a odd thing. But since you were together as a group, other possible causes could be sun poisoning issues, mishandled food issues, booze issues or even a flu that anyone in the group may have brought with them. The common sharing of a spliff can make everyone sick if just one person has the flu. Even if you dont partake, you can still pass the flu through other means.
And since the gag reflex is automatic, for each of the snorkelers to actually ingest enough saltwater, which is not the tastiest thing in the world, would be rather rare occurrence unless the group is rather small and inexperienced.
