Atoll lagoons occur when a reef surrounds an island and the reefs grow upwards as the island itself gradually subsides. When only the reefs are above sea level, the atoll lagoon is formed. As you can see here in Moorea, the lagoon actually looks like part of the island.
4. Maupiti, Society Islands (French Polynesia)
A coastal lagoon is usually when the barrier that separates the lagoon from the sea is formed and strengthened due to the movements of sand through the sea currents. -