The woman was pulled aboard a lifeboat shortly after 6.04pm and taken to the beach so that a crew member could undertake CPR on her.
An ambulance crew was waiting on the beach and took over but pronounced the woman dead.
Police were called to Brighton Road, opposite Splashpoint, just after 6pm, after reports that a body had been seen floating in the sea.
The Shoreham inshore and all-weather lifeboats were launched at 6.04pm and located the woman 50 metres from the shore at Splashpoint.
An eye witness described all those who attempted to save the woman as ‘heroes’.
The all-weather lifeboat stood off ready to assist the inshore lifeboat if needed.
There are no suspicious circumstances but the cause of death is being investigated. The coroner has been informed.