Related Clues:

• Reef in a ring
• Ocean obstacle
• Castaway's site, perhaps
• Key ring
• Sea hazard
• Lagoon formation
• 6-Down is part of it
• Snorkeling destination, perhaps
• Lagoon former
• A kind of knot
• Boating hazard
• Reef around a lagoon
• Island with a reef
• Great Barrier, e.g.
• Coral island