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 “