The man, believed to be 64 years old, went missing about 16 miles south east of Portland while on a dive trip with the diving catamaran Skindeeper.
The skipper alerted coastguards shortly before 11:00 BST on Friday when the diver failed to surface.
The coastguard confirmed a "highly concentrated" search by lifeboats, a UK Border Agency vessel and a Fisheries Protection plane was unsuccessful.
Dorset Police are liaising with the diver's family.
The 17-year-old man, believed to be from Derry, fell into the water at Rathmullan pier, Donegal at about 0130 BST on Saturday.
Irish police, the coastguard and lifeboat crews have taken part in overnight searches along the lough.
The youth's family are keeping watch at the pier where he was last seen.
Joe Joyce from Lough Swilly RNLI said an extensive search was carried out overnight fro Rathmullan to Ramelton and Letterkenny.
The coastguard helicopter joined in.
"Conditions were not great. There was a lot of cloud cover and mist. We did a number of parallel searches along the lough using flares to illuminate the area," he said.
"The coastguard and the helicopter were there using infra red technology to detect the person.
"We searched all night."
A diving club and local people are helping look for the missing man.
"I believe it was just a birthday celebration that went wrong," Mr Joyce said.
"I know the family are there on the pier keeping watch."
Devon and Cornwall Police say Joel Andrews' disappearance appears to be out of character Friends and family of a man who went missing from a rave on Dartmoor have joined a police search for him.Joel Andrews, 24, of Caldicot, Monmouthshire, was last seen at the Fernworthy Reservoir area near Chagford in the early hours of Sunday.
Specially trained officers from Devon and Cornwall Police's Tactical Aid Group and Local Policing team are searching the area he disappeared.
Police said his disappearance was described as out of character.
Mr Andrews was wearing a grey Animal hooded top and jeans and a dark green waist-length jacket.
Det Insp Brett Mitchell, who is leading the inquiry, said: "We now know that Joel was last seen at midday on Sunday 22 May and he was still at the site where the rave was held.
"I would urge anyone who attended and may have seen Joel to please contact police as they may have a vital piece of information that will allow us to find him."
His cousin, Emma Scott, said it was "really nice that people are putting in the effort".
She said: "Everybody's pulled together to help us try to find him.
"The more coverage we can get the better, because we just want to get him home now."
A Facebook page has been created by friends who joined the search for Mr Andrews.
Women and children were warned not to approach William Cunningham A registered sex offender has gone missing after he failed to attend a court hearing.William Cunningham, 44, from the Newmilns area of Ayrshire, was last seen at his home on Friday 13 May.
The alarm was raised when he failed to turn up to court on Monday. However, he has been in touch with family members.
Det Insp Neil Robertson, of Strathclyde Police, said people should not approach him, especially women or children, but should contact police immediately.
Mr Cunningham was described as being Scottish, 6ft tall, of medium build, bald but with a grey/brown handlebar moustache and sideburns.
At the time he was seen he was wearing biker clothing - jeans, black boots, leather waistcoat and leather jacket, with "Bogeyman" in red writing on the back.
Det Insp Robertson said: "I would urge that anyone with information regarding the whereabouts of Mr Cunningham to contact Strathclyde Police, however, I would advise that no one approaches him, especially woman or children, but rather contact police immediately.
"I would also suggest that Mr Cunningham give himself up to police."
It is thought that Mr Cunningham may possibly have travelled to Northern Ireland.
