It helps to understand what seaworthy means
Seaworthy isn't an absolute, it's relative. An inflatable dinghy can be seaworthy...a 300 ton yacht may not be, or either may be seaworthy within limits. The dinghy is quite seworthy for harbor use, but not for a circumnavigation...the 300 ton yacht. if badly damaged, after minimal repairs it may be ruled seaworthy to continue on its own bottom in coastal waters to the nearest port where repairs can be completed, but not seaworthy to return to its home port across the Atlantic. So your real question has nothing to do with seaworthiness, but about suitability for the use to which you want to put it. If you're planning a circumnav, is it rigged and equipped to withstand what the Southern Ocean can subject it to...if capsized, is it watertight ('cuz you might have to live in it upside down for several days). At the other extreme are nice, comfortable user-friendly (ie "forgiving") coastal cruisers--and there are lots of those, and most of 'em can make it across the Atlantic if you have the skills to do it...size is the most limiting factor for most sailors, although plenty of highly skilled and experienced offshore sailors do it regularly in boats under 30'. In between there's a full range, just as there is in cars--a Plymouth Neon is just as "roadworthy" as a BMW...it just depends upon what you're looking for in a boat, and how much you're willing to pay to have it. But when it comes to "seaworthiness," a Hunter, Catalina or Beneteau--and every other popular production boat too--is just as "seaworthy" as an Island Packet or a Swan or a Hinckley.