What kind of merchant ships would be useful in a modern naval conflict? 
 Two broad categories of merchant ships would likely be most useful in a modern naval conflict: large roll-on/roll-off ships with double hulls and small, fast ferries. If an adversary can find a ship on the open ocean and target it with a modern anti-ship missile, this opponent can certainly destroy the complex machinery needed to operate a container port or an oil terminal. Consequently, any cargo you want to unload in a war zone is going to need to drive off a ship on the back of a truck....quickly. An oil tanker will be useless, but a RO/RO ship loaded with tanker trucks can unload lots of fuel under austere/wartime conditions. Similarly, a container ship will be of little use without the large, land-based cranes used at most container shipping ports. A ferry loaded with trucks hauling shipping containers can quickly load and unload without the need for specialized equipment. Data from the Iran-Iraq war suggest that a large, 50,000+ ton RO/RO with a double hull is likely big enough to survive a hit from a modern anti-ship missile. A fast ferry that can move at ~40 knots is probably going to be too small to detect from space or shore using backscatter radar. Conversely, a 100,000-ton PanMax oil tanker or a 5,000 TEU container ship will be easy to detect, and slow enough to target.