We dressed up as indians and dumped boxes of British tea into this body of water to piss off the Brits back in the 1700s. Now we have our own tea. The Charles river outlet is what forms the harbor, but if you go a bit inland, upstream from the shipping channels, you will find lots of recreational space. Many of the universities have boathouses along the river, and registered students can take a sail boat out for the afternoon. The river basin channels more than just water.
Wind blows across the passage too, making windsurfers a fairly common site.
With their thermal wetsuits, they're on the river from mid-spring to the end of autumn. The Boston Common and the Boston Public gardens are public parks situated in the approximate center of the city. There's large sections of both shade trees and open field space. And with that Boston wind, spring and fall are prime times for kite flying. The Public gardens offer boat rides around the central lagoon all summer long.