We haven't tried this yet ourselves as we usually SCUBA dive there, but friends on a cruise a couple of years ago did.
They took a cab directly to Cemetery Beach, and arranged for the cab to come back for them. They said the snorkeling was excellent, and the cab very convenient.
Georgetown is always a zoo, and I'd be inclined to use a cab rather than a bus, and skip the shopping in favor of the beach.
Having had no experience with the busses, I can't really comment on them. Maybe they're OK?