South Shore YMCA leaders said the two-floor, 118,000 square-foot facility, set to open Dec. 2, is a much-needed upgrade from the old branch, which has been closed since a burst pipe flooded the basement on Sept. 3. Among the new building’s features are a 30,000 square-foot field house with two basketball courts and a turf field, a spacious aquatics center, and a second floor almost entirely devoted to health and fitness.