A seaworthy boat or ship is in good enough condition to safely be taken on an ocean trip. The Titanic, famously, proved not to be seaworthy.
As you drive your car onto an enormous ferry bound for Martha's Vineyard, you might worry a little about whether it's seaworthy — those cars are heavy! When something is seaworthy, it's good enough, or worthy, to travel on the sea, even during stormy conditions. This word dates from the early 19th century. The Old English version was særof, "hardy at sea."