Stumpfield Marsh appears to be the result of the Hopkinton Dam project. The pond is littered with submerged and protruding tree trunks. The whole lake/pond is shallow and the water was clear when I went in early June. It took me about 2hrs to paddle around it. The only thing to watch for is the submerged trees and your hull.
I really enjoyed it, it was breezy out but the pond is narrow so there was no effect. There's a great park on one end with a beach for kids, picnic area, playground, etc. There's a $1.00 / person park entry fee if you put in there.
The park has bathrooms, picnic areas, a swimming beach, and nature trails. The exit 5 put-in has nothing.
Park has a 1.00 / person entry fee, but as well maintained as the park is, it's worth it.
There's two places to put in. #1 take I-89N to exit 5. Go about 1 mi and you will see a boat ramp sign on the right. #2 take exit 6, turn left. You will see the park sign about 1 mi down on the left. (a good place for the family)