Steel has an unfortunate habit of rusting when mixed with water. Navies can afford the manpower to keep the metal protected with layers of paint and to buy new when they deem it advisable. Most other owners follow less stringent maintenance & replacement schedules... IMHO that oil tankers - like the Prestige - typically have a life expectancy of about 30 years because of rust eating through and weakening their plating. Cruisers seem to like steel hulls because they expect to ram them onto coral, and the steel holds up better in that situation than does fiberglass. Steel seems more popular in Europe than around N.America, possibly because more people see steel barges on the rivers that flow through central business areas, and hope to head to those coral-atoll-colonial outposts. Note, however, that the barges are mostly traveling in fresh water.