Charles River scullers
It looks like they may have been getting ready to film this group of sculling rowboats going down the river. Just visible to the right in this shot is what appears to be a video camera boat, while in the distance downstream was a police boat. I thought they were sending down divers looking for something, and the rowers were being told to wait, but in hindsight the simpler (& happier) explanation was that someone was making a movie of the boats going down the river.
Supposedly Justin Timberlake is filming The Social Network about Mark Zuckerberg and the founding of Facebook in Boston these days, and I thought I heard them mention Timberlake's name. I only sort of know what he looks like, so for all I know he or some other actor(s) are in these sculling boats. Me, I just like seeing the boats rowing down past the autumn foliage, nevermind who is rowing them :-)