The Thames is Britain's best-known river, immortalized in the novels of Charles Dickens, and a waterway once so putrid it shut Parliament because of its stench. It's now getting the Olympics treatment because of its potential to give would-be terrorists a foothold.
"There's no specific threat but we would be failing in our duty if we didn't consider it," said Chris Allison, national Olympics security co-ordinator. He said forces were testing methods used to force vessels to stop and practising boarding-at-sea procedures.
A Navy helicopter swooped overhead as crafts moved around on the Thames, known for its strong currents and tides, as London's Marine Policing Unit and Royal Marines worked to make sure the police and military know how to work together on the river that snakes some 338 kilometres across England.
Policing the vast territory for the games in July and August won't be easy. The security detail around the Olympics has grown to involve the military and hundreds of security guards.
The tests this week have involved some 44 police officers and 94 military personnel in the run-up to the games, which start July 27th and end Aug. 12th.
Some recent terror attacks like the Mumbai shooting spree in 2008 were launched from virtually unpatrolled waterways. Terrorists in the Indian metropolis arrived on small high-speed boats prior to their deadly attack that killed 166 people.
British authorities want to make sure the same type of attack doesn't happen in London. Security officials have expressed concerns over would-be attackers using the river as a means of access into the games, but they are also concerned attacks could be launched against London's financial centre, which sits close to its banks.
Security officials say they also worry about scares away from the Olympic Village that could divert resources away from a real attack.
Security has long been a high priority for the Olympics. A terror attack at the 1972 Olympics in Munich killed 11 Israeli athletes and coaches.
And London was the first European capital to be struck by suicide bombers in 2005 — the day after the city won its bid to host the games. Some 52 commuters were killed during the rush-hour attack in July 2005.
About 12,000 police officers will also be on duty on the busiest days of the games — together up to 13,500 troops deployed on land, at sea and in the skies. A huge presence of private security guards will also safeguard Olympic venues.
The Thames is a less than enticing vantage point for the Olympics — even for would-be attackers.
Although it has been cleaned up since the "Great Stink" of 1858 that forced the House of Commons to stay at home, its muddy reputation precedes itself. Comedian David Walliams battled diarrhea after doing a charity swim along the River Thames last year.
The journey is almost seven times the distance of the English Channel and is associated with the Victorian era novels of Dickens, such as "Bleak House."