The bodies of a 17-year-old girl and 18-year-old boy were found at Bothenhampton Reservoir, near Bridport, on Tuesday morning.
Dorset Police have said that the deaths are being treated as non-suspicious but investigators are now trying to establish the circumstances on behalf of the coroner.
The force is now seeking anyone who may have seen the two teenagers in the Bridport or West Bay area between Saturday January 22 and Tuesday January 25.
The male is described as white, around 5ft 7in, of slim build and with short, dark hair, a short beard and brown eyes.
He was wearing brown lace-up boots, light grey tracksuit bottoms, a dark T-shirt and a navy hooded Parka-style jacket with a brown patch on the left arm.
The female is described as white, around 5ft 5in, of slim build with dark brown, shoulder-length hair and brown eyes.
She was wearing black and white trainers, light grey tracksuit bottoms a navy Puffa-style jacket and a pink scarf.
Detective Sergeant Simon Lee, of Dorset Police, said: ‘Our thoughts are with the families and friends of the two young people who tragically died.
‘As is normal in these cases, we are now making inquiries on behalf of the coroner’s office to establish the circumstances of the deaths.
‘Therefore, we are appealing for information from the public to help us piece together the two young people’s movements prior to their tragic deaths.’
He said they may have been carrying a pale green Nike bag.
DS Lee added: ‘Anyone who thinks they might be able to assist with our inquiries is asked to contact Dorset Police.’
The identity of the individuals is not being released until an inquest is formally opened by a coroner on the request of the families, Dorset Police said.
The force has referred itself to the Independent Office for Police Complaints (IOPC) over the deaths due to prior contact with the individuals.
The IOPC said on Thursday: ‘We are currently assessing the available information to determine what further action may be required from us.’