Britain's only prison ship should be closed unless an expensive facelift is carried out, a report says.
Ms Owers said HMP Weare is "entirely unsuitable for its function"
Chief Inspector of Prisons Ann Owers said HMP Weare, off Portland, Dorset, which opened in 1997, was "unsuitable, expensive" and "in the wrong place".
Ms Owers made 116 recommendations for improvements at the 400-capacity jail.
Area Prison Service manager Jerry Petherick said an appraisal was being carried out on the facility but no decision on its future had been taken.
Ms Owers' report said prisoners have "no access to fresh air".
"[The prison] has no space for workshops, and insufficient space for exercise or education," she said.
"Despite the best efforts of staff or managers, HMP Weare is entirely unsuitable for its present function as a 21st Century category C training prison."
Speaking on BBC Radio 4's Today programme she said: "If you look at the cost benefit the inevitable conclusion would be that it should close.
"It would be a much more effective, and cost effective, to have quick built units in other training prisons."
One inmate, 31-year-old Paul, said he was lucky to have a sea view cell.
"The only bad thing is we don't get any air," he said.
The HMP Weare, is moored off Dorset's Portland Harbour
"I get regular headaches through lack of air."
Prison governor Denise Calvert said inmates had one hour's exercise in the yard and one hour to use the gym facilities every day.
Ms Owers said "Millions of pounds of capital investment would be necessary to make it more suitable - indeed, even to keep it seaworthy and safe will require significant resources.
"But even then, it would be a training prison in the wrong place, with limited facilities and in a constricted environment.
"Weare is, literally and metaphorically, a container," she added.
Accommodation in the healthcare centre was described as "very poor" and jobs for inmates were said to be "very limited" in the report, which was published on Thursday.
Ms Owers made 116 recommendations for improvement at the jail.
However, she did note the "good staff-prisoner relationships" and some resettlement work being developed.
Phil Wheatley, Prison Service Director General, said: "HMP Weare was opened in 1997 as a temporary solution to a sharply rising prison population and we agree it is unsuitable in the long term as permanent accommodation.
"A decision concerning its future will be made as soon as possible."