Prisons are expensive. States lock up only as many people as it takes to keep voters from getting up in arms about crime, and not one more. A small used container ship can be bought for $5 million. 40-foot shipping containers cost around $1,250. Fitted with plumbing, wiring, insulation, drywall, ductwork, windows and doors, each container becomes a spartan housing unit for roughly $8,500. Bunks stacked three-high in two rows along each side, one container would sleep about 30 inmates, and still have room for a shower, toilet, and sink. A small ship holds about 250 such units, enough for...