(CNN) -- Before luxury yachting was the preserve of Russian tycoons and Silicon Valley moguls, it was only the world's wealthiest royals who built palaces on the sea.
There have been and continue to be a fleet of imperial yachts used to transport royals, from Russian czars to princes of Monaco, in the opulent fashion to which they are accustomed.
There are probably few finer examples of a regal leisure boat than "Her Majesty's Yacht (HMY) Britannia," built in 1953 for Britain's Queen Elizabeth II.
Now decommissioned and on display in Edinburgh, Scotland, the vast and lavishly designed "HMY Britannia" has sailed over one million miles during 44 years of service and in the course of 968 official royal visits.
Once described by Queen Elizabeth as "the one place where I can truly relax," the royal yacht, built to many of the queen's specifications, boasts huge dining rooms adorned with gifts and curiosities from around the world, including a whale rib found by her husband on a beach.
In addition, there's a sun lounge with furniture chosen by the queen and a garage built to house the royal Rolls-Royce.
Sir Winston Churchill, Boris Yeltsin, Rajiv Gandhi and Nelson Mandela are among the famous names who have joined the queen on board over the years, but "HMY Britannia" was also deployed for more private and romantic occasions.
As Kate and William add the finishing touches to their wedding plans, they may well feel a pang of regret that "Britannia" is no longer in service. Four royal honeymoons took place on board, including Prince Charles and Princess Diana's 16-day trip in the Mediterranean.
Today, the undisputed champion of royal vessels belongs to Dubai's Sheikh Mohammed bin Rashid Al Maktoum, whose $300 million mega-yacht is christened -- somewhat unimaginatively -- "Dubai."
Measuring 524 feet long, it's the world's second-largest yacht, eclipsed only by Russian billionaire Roman Abramovich's suitably named "Eclipse."
Three elevators and one vast open glass stairway serve its multitude of decks which, aside from the gold-finished, jewel-encrusted VIP guestrooms and appropriately palatial master bedrooms, contain a squash room, spa, banquet hall and cinema. And did we mention the helicopter pad up top?
In keeping with the fantasy-flavored extravagance of "Dubai," the yacht is moored year-round on the artificial "Logo Island," situated next to the country's emblematic, man-made archipelago "Palm Islands" -- built from one billion cubic meters of rock and sand.
If you thought that Abramovich and his fellow billionaires were the first of their countrymen to build ultra-ostentatious pleasure boats, then think again.
The Russian imperial yacht "Standart," built according to the specifications of Emperor Alexander III and his son Nicholas, was the largest imperial yacht on the oceans during the late 19th and early 20th century.
Completed in 1895, the opulent vessel was 401 feet long -- about the length of a soccer pitch -- colossal even by today's immodest standards.
Indeed, "Standart" was a veritable floating palace, adorned with mahogany-paneled drawing rooms, formal salons with polished floors, brass fittings, crystal chandeliers and velvet drapes.
The czar's private study was furnished in dark leather and elegant wooden furniture, while the czarina's drawing room and boudoir were bedecked in her favorite English chintz. The imperial yacht even had its own chapel for the private use of the family.
However, Russia's largest royal yacht was also her last. After the revolution in 1917, the ship was stripped of all its elegance, renamed "Vosemnadtsate Martza" and refitted as a drab, gray minelayer for service in the Soviet Navy. The boat was scrapped at Tallinn in Estonia in 1963.
Decked out with a Jacuzzi, sea kayaks, snorkeling gear and wetsuits, "M/Y Grace" is one of the few specially tailored yachts fit to chart the delicate waters surrounding the Galapagos Islands.
The boat's regal past is only hinted at in its name. "M/Y Grace," as the vessel is now known, was once the royal yacht of Prince Rainier and Princess Grace of Monaco.
When American actress Grace Kelly married the prince of Monaco, the couple were extensively pictured honeymooning on this exquisite 147-foot-long yacht, given to them as a wedding gift by prominent Greek shipping merchant Aristotle Onassis.
The boat was captured on boundless newsreels as cameras and reporters followed Kelly on her last voyage from the United States to her new home in what was then styled "the wedding of the century." The couple left the day after the ceremony to cruise the coasts of Corsica and Sardinia.
The K/S Norge is one of the last active royal yachts in Europe, belonging to Herald V, King of Norway.
Built in 1937 by Camper and Nicholson, the oldest leisure marine company in the world, the boat was originally owned by British aviation pioneer Sir Thomas Sopwith -- who had given it over to the UK's Royal Navy to serve as a convoy escort vessel during World War II.
In July 1947, the ship was purchased by the people of Norway as a present to the much beloved King Haakon VII for his 75th birthday. The royal yacht was renamed "Norge," the Norwegian word for Norway.
Still used today for state visits abroad, the classically shaped yacht is also employed as a base for the king when he competes in international yacht races.