Detective chief inspector Mike Ashcroft said: "Although we can't categorically prove that Mark is dead at this time, we are treating this as seriously as we can.
"His sudden disappearance is completely out of character. He is close to his family but they have not heard from him at all."
Mr Ashcroft added: "There were no signs that he was depressed or may wish to harm himself. He seems to have vanished.
"His family are desperate to know what has become of him and they are being supported by a specially trained police officer.
"We want to hear from anyone who may have seen Mark or has any information about where he may be."
Mr Manning is white, 5ft 9in tall, clean shaven, with a tanned complexion and is of stocky build. He has blue eyes and is bald.
More top news
Friday morning's weather for the East of the region
Friday morning's weather for the West of the region
Five men who died in the water off Camber Sands were friends who had arrived to spend the day at the beach together, ITV News understands.