Where are the nearest fitness centers or gyms in Central Waterfront, San Francisco?

There are actually a ton of fitness trainers and fitness centers in Central Waterfront, San Francisco and in neighboring Potrero Hill. The two main recommendations I would have would be Fitness for the Heart and I Luv My Body Fitness. But you can also find a number Zumba trainers around and Cafe Cocomo is here as well (for Salsa Dancing at night), so you are completely set in terms of fitness in the area.