What is meaning of seaworthy ship/when a ship is called seaworthy?

A vessel is seaworthy if the vessel and all of its parts and equipment are reasonably fit for their intended purpose [and it is operated by a crew reasonably adequate and competent for the work assigned].legally defined, a seaworthy ship is one that is fit for any normal perils of the sea, including the fitness of the vessel itself as well as any equipment on it and the skills and health of its crew. 

Note that this only includes the perils of the sea, as opposed to the perils on the sea, and so does not include piracy, severe storms or other such hazards that may occasionally be encountered.