Simpson Bay Lagoon.

As the Caribbean’s largest landlocked lagoon, Simpson Bay Lagoon is the bustling heart of Saint Martin’s maritime scene. Split down the middle by the French-Dutch border, this massive 12-square-mile saltwater basin is a working harbor rather than a sleepy backwater. Yacht-watching is a major draw here, especially when the lift bridges on either side open on schedule to let mega-yachts pass through. While the shoreline is lined with busy marinas, repair yards, and waterfront bars, the lagoon also harbors ecological treasures like the protected red mangroves of Mullet Pond. With low-flying planes from the nearby airport roaring overhead and a constant parade of boats on the water, it is a dynamic, highly social hub that defines the island’s active coastal lifestyle.