Two sides of the Parish of St Osyth are flanked by water. To the northwest is Flag Creek, to the west is River Colne and to the south is the North Sea. Branching off Flag Creek and running eastwards towards the village is St Osyth Creek.
Much of the coastline is saltmarsh, mud and sand providing a valuable habitat for nature, particularly migrating waders and waterfowl. Shingle beaches provide a nesting area for many land birds and the saltmarshes contain rare plants and insects. A seawall generally lies to the rear of the saltmarsh and creates areas of fresh water marsh grazing behind it. The area is considered of international environmental importance.
St Osyth Creek
St Osyth Creek is a two kilometre long tidal creek running eastwards from Brightlingsea Creek to a causeway across the valley where an old tidal mill once existed.
Near the site of the Mill is St Osyth Boatyard, a mixture of house boats, moorings and hardstands for boat storage. The boatyard specialises in the restoration and maintenance of traditionally built wooden boats old and new.
Beyond the causeway is the 800 metre long Mill Dam Lake.
Flag Creek is a 3 kilometre tidal continuation of Brightlingsea Creek. The industry which once occupied its banks has now gone. The brickworks are closed and today are replaced by static caravan parks. The sand and gravel extraction has also ceased although the barge loading gantries still stand as if awaiting a revival.