There was a time when no monarch worthy of his ermine considered a throne worth sitting on unless its perquisites included a private yacht. But no more. Frederika of Greece, whose royal veins course with the blood of a host of Europe's kingly houses, has a throne but no yacht. Most of her royal cousins have neither. Then Frederika got an idea: she and her husband, King Paul, would play hosts to their less fortunate relatives aboard Greece's brand-new 5,500-ton liner Agamemnon. Gratefully, the members of Europe's royal families swept aboard the ship at Naples.
Numbering nearly 90 in all, they were representatives of the present ruling houses of Greece, Holland, Luxembourg, Norway, Denmark and Sweden; disinherited princelings from Italy, France, Spain, Rumania, Yugoslavia and Bulgaria; dynastic relics from kingdoms whose thrones had long since ceased to exist: Bourbon-Parmas, Mecklenburgs, Schaumburg-Lippes, Hesses, Thurn und Taxis, and Hohenlohe-Langenburgs.
Since they were all related in one way or another, making the trip a family party, Cousin Frederika ordained that everything should be informal. Only 85-year-old Prince George of Greece (whose Bonaparte wife is one of France's leading psychoanalysts) was allowed to bring along a personal servant. Formal dress and court protocol were forbidden, and the seating arrangements for each day's meals were drawn democratically by lot.
The Agamemnon's captain and first officer moved into second-class quarters to make room for the Greek King and Queen. The Grand Duchess of Luxembourg parked her duds in the chief engineer's bunk. Ex-King Michael of Rumania and his honey-haired wife Anne were berthed in a double stateroom.
As the cruise ship nosed its way through the Isles of Greece, stopping daily to give the tourists a chance to see the sights by bus or on muleback, royal teen-agers hacked around like any other kids, squirting each other with pop, staging impromptu Olympic games in ancient stadia and rewarding winners with stolen kisses. In the evening there were movies, and sometimes all hands joined to practice the mambo and the rumba, with Frederika easily carrying away top dancing honors. While the youngsters gulped gallons of Coca-Cola, their elders forsook champagne in favor of solider Scotch. At the end of one hilarious evening, some of the more enthusiastic princelings tossed their cousin Christian of Hanover into the ship's swimming pool fully clad, then all jumped in themselves. After that, Frederika ordered the pool emptied each night at 2 a.m.
Last week when the Agamemnon docked at Naples once again at the cruise's end. it was generally agreed that the trip had been a huge success, and as Don Juan, pretender to the throne of Spain, put it, a bit forlornly, "a fine chance for the children to get to know each other."