A Prison ship?
You mean we can just build a ship and put them on it. Send them out to sea?
Prison ships were usually old dismasted “hulks” that were anchored in harbors. Hard to escape from, impossible to tunnel out of.
Docked off the Bronx in Long Island Sound, the Vernon C. Bain Correctional Center prison barge is recognized by the Guinness Book of World Records as the largest operational prison ship in the world.