It's an idea I had a while ago: take 9 old oil rigs and connect them in a 3x3 grid with secure covered walkways within a rigid frame to securely hold all 9 in place with the centre one turned into a communal open area and one used as the entrance/prisoner and delivery processing/guard and staff accommodation. Build the rest up into prison blocks and once moored so far out into the Atlantic that escape becomes suicidal and hey presto, secure prison.