Osea Island, formerly also Osey, is an inhabited island in the estuary of the River Blackwater, Essex, East England. It is approximately 380 acres (1.5 km2) in size and is connected to the north bank of the river by a causeway that is covered at high water. The island's population is included in the civil parish of Heybridge.

Mersea Island is an island in Essex, England, in the Blackwater and Colne estuaries to the south-east of Colchester. Its name comes from the Old English word meresig, meaning "island of the pool" and thus is tautological. The island is split into two main areas, West Mersea and East Mersea, and connected to the mainland by the Strood, a causeway that can flood at high tide.