Cheese.
Despite harsh working and living conditions Foulness Island was a prosperous area from a very early time due to its rich coastal grazing marshland. This is evident as each of the mainland parishes of Sutton, Rochford, Shopland, Little Stambridge and Little Wakering had shares in the grazing pastures and its produce. As well as dairy produce, which was the most important, the sheep were also prized for their skins, wool and meat. The earliest records are from bailiff accounts of 1420 and 1424 which detail acreage and production of many of the wicks present on the island, plus also those of Canvey and Wallasea Islands, which shared similar prosperity. In total, the three islands were home to upto 18,000 sheep, the benefits of which were shared by 18 local parishes and manors in total, with produce being exported to all parts of England and also abroad. Detailed records of land use and production exist since this time.
During the mid 16th century, the reformation was responsible for the manorial decay and change of land usage, so gradually ownership was shifted to local tenants and farmers. Agricultural changes also started to shift towards focusing on arable farming and slowly dairy production began to decline and by the mid-eitheenth century arable farming overtook dairy.
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We got all of this info from John. There is a VERY detailed book available from the Essex Record Office.