The police, coastguard and a lifeboat crew spent nearly two hours searching the sea off Brighton after a swimmer was reported missing.
A spokesman for Brighton and Hove City Council said: “The search was started at 10am after a swimmer reported seeing a swimmer with a white swimming cap enter water but not leave.
“A bag was found on the beach that the seafront office suspected may belong to the swimmer.”
Shortly before midday the search was called off with those taking part having been unable to find the missing swimmer.
appealed for anyone with information about the swimmer or the bag to contact the force on 0845 6070999.
The council spokesman said: “We want people to enjoy the sea but do tell someone when you go swimming and when you will be back and don’t leave items on the beach unattended.
“If you do, write a note on them in case they trigger a search.”