MANHATTAN'S younger social set is having to do without its favorite couple, Crown Prince Pavlos of Greece and his bride, Marie-Chantal Miller. The two are on an extended Pacific cruise in her dad Robert's luxurious yacht, the Mari-Cha 111. Pavlos just helped sail the boat in Australia's tough Sydney-Hobart race and has now headed back up the Down Under coast to the glorious Whitsunday Passage for some all-over tanning. Then they head for New Zealand to watch the February finals of the America's Cup, yachting's oldest contest.