Working out in the pool is probably the best method out there to effectively and safely lose belly fat and build six pack. But that doesn't mean its an easy walk. Still, working your core muscles in a safe buoyant environment with your body supported from all sides is far better than doing it on a dry land.
Don't expect fast results though. There is no magical formula here. The area around our belly is considered a major fat storage place, especially for men. This puts a lot of challenge on your task. In order to keep up with your body's continuous storing of fat you will have to exercise on a regular basis and cut down on fatty food.