In the estuary/lagoon behind a barrier island, you would NOT expect to find a coral reef.
Coral reefs typically thrive in warm, clear, and shallow ocean waters where there is significant sunlight and relatively stable salinity, conditions that are usually found in tropical regions. Estuaries and lagoons, on the other hand, are often characterized by varying salinity levels due to the mixing of freshwater and saltwater, which are not conducive to coral reef development.
The other listed ecosystems—mudflats, seagrass beds, mangroves, and oyster reefs—are commonly found in estuarine environments.