They close the pools at night so they can drain them and clean them. I don't recall the specific hours on RCCL ships, but one can usually gauge the pool hours with the pool bar hours. Most of the time, the pool bar on RCCL ships stays open until 9pm. Hopefully someone with a better memory of RCCL cruises will chime in here. I looked over some old Cruise Compass copies from our past RCCL cruises and could not find any mention of the pool hours.