Widewater has special SNCI (Site of Nature Conservation Importance) status as a unique saline lagoon, and is the subject of an Action Plan by the Sussex Biodiversity Partnership. The brackish lagoon was formed in about 1840 when an embankment was constructed on the south side of the coast road. The shingle bank to the south was formed by longshore drift, which extended across the mouth of the River Adur and formed the present harbour entrance.

All of the northern embankment has been developed. The southern, shore-side embankment is grassy with clumps of wind-stunted shrubs such as Elm and Hawthorn. Among the herbaceous plants growing in the well-drained, exposed conditions are:

Teasel · Coltsfoot · Burdock · Smooth Hawk's-beard · Prickly Oxtongue · Sea Mayweed · Pineapple Mayweed · Common Fleabane

Incoming autumn migrants appreciate the clumps of Fleabane on the embankment as their first meal on British soil after the Channel crossing.

In August 2000 a large influx of Clouded Yellow butterflies appeared in Shoreham and Lancing. Many were sighted on the vegetated area of the lagoon embankment. Similar sightings were reported across much of southern England that season.