Snorkeler's destination (4)
(Other definitions for reef that I've seen before include "Type of knot" , "The sort of knot" , "Undersea ridge; reduce sail area" , "marine hazard" , "The Great Barrier ..., Australian natural wonder" .)
(Other definitions for atoll that I've seen before include "Bikini, say" , "Coral isle" , "String of islands" , "Ring of coral" , "tropical island" .)