Snorkelers are thrilled with the Blue Hole trip! However, they do not take snorkelers (divers only) to Turneffe Elbow. The dive shops must take along an extra crew member if there is just one snorkeler, to stay with the person who is snorkeling. They will do it for the Blue Hole trip but not the other "day" trips.
Different guides are qualified for diving, so you can't "always" snorkel at the same time someone else is diving.
Hol Chan and Shark Ray Alley is another spot where dive shops offer a "combo" trip to divers. They snorkel at one stop and dive at another. Generally, a snorkeler can go on these trips as well.