Marcel, who is 15, was in the Cobden Bridge area of the city at around 11.55pm yesterday.
He was spotted entering the water and he has not been seen since.
He is described as white, slim, around 5ft 9ins tall, with short brown hair. Marcel was last seen wearing a dark coat, black Nike Air Jordan trainers with red markings, grey tracksuit bottoms and a black rucksack.
Detective Inspector Howard Broadribb, said: ‘Officers remain on-scene today conducting searches and further enquiries, but we are very concerned for Marcel’s welfare, and are asking for the public to report sightings of him to us.
‘It may be that he had made his way into a local garden, outbuildings or in boats given that he was last seen close by the marina. We would ask the public to check these areas for any possible sightings of Marcel.’
If you have seen him, or know of Marcel’s whereabouts, please call 101 immediately quoting 44210447804, or dial 999 in the event of an emergency.