On Serenade, the pools were open until 7pm, but nearly everyone is at dinner, at a show or elsewhere, so there's no point really in keeping the pools open 24/7. Plus, too dangerous. Once false trip by a drunk who falls into the pool & dies since nobody watches the pool & then the cruise line is blamed & law suit ensues.
Not worth it.
Keep the salt water! Salt water kills bacteria (hence, anytime you get Poison Ivy, people always tell you to put it in salt water). It will make up for the disgusting people who don't wash their hands when they leave the bathroom. At least when they go in the pool, they will be sanitized.