most underrated swimming spots close to boston

I live in boston and I am a BIG fan of swimming. Currently I love Crystal lake in newton because it’s green line accessible and the water is lovely. When I have access to a car, Walden pond is fantastic. It can be busy but full of private little alcoves where you can set up and relax by the crystal clear, deep water. What are your faves? The wilder and more seclusive the better. Bonus points for being relatively close to the city.