’’ It’s the best kept secret in this city,’’ says John Chiesa, manager of the Medford Boat Club, which is smack dab between the Upper and Lower Mystic Lakes. If Everett is the beast of the river, this is the beauty. There is water-skiing, tubing, water slides and beach. ’’I always say that if we blindfolded you and took you on a two hour ride and said, ‘Where are you?’ . You’d say Vermont or New Hampshire. That’s the true beauty of the place.’’