Related Clues:

• Shoal
• Knot
• Coral ___
• Nautical danger
• Reef
• Island ring
• Sailor's peril
• A-test site, perhaps
• Bikini, e.g.
• Coral ridge
• Ring around a lagoon
• Lagoon perimeter
• Reef, maybe
• Site of a wreck
• Wake Island, e.g.