Salt tidal waters enter the Fleet from the west and other sea water seeps through the shingle of the bank. This mixes with fresh water from inland streams making for a brackish lake the salinity increasing from the west, where the nominal depth is one third of a metre, to the east and a depth of 5 metres.

Abbotsbury was once the site of a monastery and a swannery has existed here on the Fleet for centuries, a Keeper of Swans being recorded in 1393. With the dissolution of the monasteries by Henry VIII the lands passed to the Strangeways family who have maintained the swannery ever since. They also built an important decoy to trap ducks for the table. Today the decoy is used extensively for ringing wildfowl to keep track of migrations.

Near Weymouth to the east is Moonfleet Manor of smuggling fame and nearer still to Weymouth is the Army training area at Wyke Regis where Royal Engineers are trained to bridge rivers. The mouth of the Fleet is at Ferrybridge which, as the name implies, was once the site of the ferry to Portland before the bridges were built in the nineteenth century.

One item of WWII interest is the use of the Fleet as a range for the Dambusters Squadron of the Royal Air Force to perfect their skills with bouncing bombs prior to their famous raids on the Roer Dams