I'd say where to go depends on what you are interested in seeing....
To give you an idea of what will be going on, here is a map of the 2007 course: hocr.org/pdf/2007_HOCR_map.p...
For rowing, we always get a kick out of viewing the races from one of the many bridges over the Charles. It's neat to see the racers shoot right under you! A big bridge, like the Mass Ave bridge, will have lots of room while a pedestrian bridge, like the Weeks Bridge near Harvard Square, might be more crowded by won't have cars. Of course, we're not rowers and never have been, so I admit that more serious spectators might not appreciate this particular strategy. ;)
In addition to the races, there are vendors selling/giving away their products, music, and lot of other entertainment along the banks of the River. In recent years this has become a larger and larger part of the weekend.
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