The slides close and the lifeguards leave at a certain time, but the pool never really closes. We've been told several times that you can swim at any time, as long as you aren't making too much noise.
During the warm months, we go for a swim after we get back from the parks at night, sometimes as late as 2:00-3:00am. The lights have always been on, and occasionally there have been castmembers working in the area, but we usually have the pool to ourselves.