Mersea Island IPA: [m3:.zi] 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". The island is split into two main areas, West Mersea and East Mersea, and connected to the mainland by the Strood, a causeway that floods at high tide.