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Yacht watchers will be on the lookout for the “Eclipse,” the 538-foot ship owned by Roman Abramovich, the Russian oligarch who owns the Chelsea soccer club; the “Octopus,” the 414-foot yacht owned by Paul Allen, the Microsoft billionaire; and the “Leander,” the 246-foot yacht owned by Britain’s Donald Gosling, who made his fortune in parking garages.

It’s not as if London has gotten suddenly more dangerous. But for someone like Mr. Abramovich, who is worth billions, his family could face security threats if they were crammed onto the Jubilee Line, a main artery to the games.

PSI offered reporters a demonstration ride on the Thames, offering a taste of how the super rich might get to their seats at the beach volleyball competition.

It started with champagne at the Savoy, London’s elegant hotel. Then a trip on a rigid inflatable boat down the Thames followed the bubbly, a zippy excursion that could give Disneyland ideas on roller coasters and white water rafting. Envision ex-Marines reliving glory days with a high-powered boat to play with — you get the picture.

PSI declined to talk about prices — saying that depends on what the client wants and how complicated the security arrangements are.

At this level, talking about money is a tad crass, anyway.

But no matter what, they offer great scenery. The Thames will provide the images of London that viewers around the world want to see this summer and will remember for years to come.

“The river is the very reason for London,” Mr. Blyth, the naval historian, said. “London exists entirely because of the Thames.”