I just got back from a week of snorkeling at Lighthouse Reef, and it was excellent! Would imagine Turneffe would be similar, due to no crowds, and less traffic damaging the reef.... Makes Hol Chan and the local snorkeling near Ambergris look like Jamaica. South Water Caye is in my future, but don't think it offers as many nearby options as Turneffe or Lighthouse. Was the only snorkeler at the resort, but was welcome on the dive boat, or could arrange snorkel trips when the dives were on deep reefs/walls. The place does not get much coverage here - but everything about Lighthouse Reef Resort was exceptional and I would go back in a minute!