A striking thing about the venues for the 2012 Olympics in London is that their locations make more sense when seen as connected by water rather than by road.
Greenwich Park (equestrian events, modern pentathlon) overlooks the River Thames at Greenwich. You can get there on foot from the Olympic Park by taking the Lee Navigation towpath to the Limehouse Cut and traversing Docklands before cutting under the river through the Greenwich foot tunnel—see map below.
It’s a nice walk, though you can’t do it directly from the Olympic Park during the Olympics because the Lee Navigation towpath is closed for “security reasons”. I hope they remember to re-open it afterwards. In the meantime you’ll have to join the towpath at Stratford High Street.
Also on the Thames, the Royal Artillery Barracks (shooting) in Woolwich are just downstream, a bend away, on the same riverbank—roughly opposite the entrance to the Royal docks, on which the ExCeL centre (fencing, boxing, weightlifting etc) sits.
Hampton Court (cycling time trials) and Eton Dorney (rowing) are on the Thames as well, much further upstream. More centrally, you can find Horse Guard’s Parade (beach volleyball) and The Mall (marathon, cycling road race) set back only a couple of hundred metres from the river.
The Lee Valley white water centre (canoe slalom) is on the same river as the Olympic Park. Presumably you could canoe between the two of them, though it might take a while.
Lord’s (archery) is accessible from the Regent’s Canal; from there you can walk all the way to the Olympic Park along the towpath via the Hertford Union Canal—apart from that pesky tunnel at Islington, for which you’ll need a boat, or to rise to street level (see map).
Wembley Stadium (football) and arena (badminton) are on a site bounded by the River Brent. An ideal way to get there from Lord’s should be to take the Regent’s Canal down to Little Venice, the Grand Union Canal from there up to Alperton, and then follow the Brent through the Tokyngton Recreation Ground. But I’m not sure whether that last connection can easily be made on foot, and the Brent isn’t navigable. I’ll have to go and take a look.
Earl’s Court (volleyball) and Hyde Park (open-water swimming, triathlon) can’t be reached by water, but both were built on top of culverted rivers or streams—Earl’s Court on Counter’s Creek, which reaches the Thames at Chelsea harbour, and Hyde Park over the Westbourne, which formerly supplied the water for the Serpentine.
The odd one out is Wimbledon, which is nowhere near any present or former waterway as far as I can see.