HMP Weare was the UK's only prison ship until five years ago
The Conservatives could bring back prison ships even though the move has not been included in their draft manifesto, the party has said.
The Tories said it was being considered as part of a pledge to end Labour's early release scheme for prisoners.
The scheme allows non-violent offenders in England and Wales to be freed up to 18 days early to ease overcrowding.
The UK's last floating jail, HMP Weare, was sold in 2005 after eight years holding prisoners off Portland, Dorset.
A Conservative spokesman said: "This proposal has not been included in our draft crime manifesto.
"But it is something we are considering as a way of ending Labour's early release scheme that has allowed 75,000 offenders to be released early from prison."
Chief Inspector of Prisons Anne Owers criticised HMP Weare at the time as being unfit for purpose, saying it restricted prisoners' access to fresh air and to exercise.
She said it was "merely an expensive container" and that it needed extensive refitting.
The Weare was originally a troop ship in the Falklands War and then a floating jail in the US.
Ministry of Justice figures released last year showed more than 50,000 criminals had been released from prison under the End of Custody Licence (ECL) scheme since it was introduced in 2007.
Ministers say the early release scheme is a carefully considered way of reducing overcrowding, but opposition parties claim it fuels crime.
A Prison Service spokesman said: "Ministers have made clear they will end the End of Custody Licence scheme as soon as there is sufficient capacity to do so.
"We are making good progress to increase prison places to 96,000 by 2014 - having increased it substantially over the last decade.
"All prisoners released on ECL would have to have been released anyway in a maximum of 18 days' time.
"Prisoners serving a sentence for a serious violent offence are excluded from ECL."