Related Clues:

• Key ring
• Lagoon formation
• 6-Down is part of it
• Lagoon former
• Hazard to navigation
• Boating hazard
• Reef around a lagoon
• Island with a reef
• Coral island
• Ringlike formation
• Bikini, notably
• It's often formed around a sinking tropical island
• Castaway's locale, maybe
• South Seas spot
• Key chain?