“United States Ship” is applied to warships (combat vessels);

Other designations include:

USNS- United States Naval Ship, these are auxiliary type ships, such as tankers, supply and ordinance vessels, as well as naval research vessels, salvage, and generally any non-warship operated by the Navy

M/V- Merchant vessel, this would be civilian, and include freighters, container ships, ferries, cruise ships and super tankers.

F/V- Fishing vessel, kind of self explanatory.

The Moondog