Lower Manhattan’s most closely-watched, once-stalled development site is “fit”  to rise. Chelsea Piers Fitness will launch a five-story, 76,000-square-foot health and wellness facility at 250 Water St., the one-acre empty lot that developer Tavros bought from Seaport Entertainment Group  last year, The Post has learned. The lease is the first of any kind for the ambitious project, which is scheduled to break ground next winter. It will boast 600 apartments, 25% of which are earmarked as “affordable,” as well as commercial uses and abundant public outdoor space. Rendering of the 250 Water St. development’s lower floors. The project is scheduled to break ground next winter. SOM/Fogarty Finger Atlantic Retail will handle leasing for 35,000 square feet of ground-floor, high-ceiling store space.   The planned development will eliminate what has long been a blight on Downtown’s East River side. Tavros and Atlas Capital bought the land for $150 million in July.…