The Pennamaquan River in Pembroke, Maine has a wildlife management area spanning the two dams on the river. My parents retirement home was right in the middle of these two areas so I am quite familiar with the site.
The short river (~8 kilometers) is marshy and home to numerous birds and animals. Alewives and eels swim up the fish ladders to spawn and I have gone swimming with a friendly otter. An old railroad bed would lead deep into the woods next to my parents farmhouse I have seen both deer and black bear on this trail.
Good swimming can be had with both a rope swing and a bridge to jump off.
Across the street from the access point to the dam is a fresh water spring with delicious cold water.