When we last sailed on the Serenade about 3 years ago, the pools were open 24 hours.
Caveat: The drain and clean them overnite, but don't start until after midnight and only do one pool at a time, so if the main pool is closed for cleaning, the Solarium will be available and visa versa (is that how that is spelled?)
When we were in Vegas I had the same disappointment. They want you gambling, not swimming.
However, again, this was years ago, Tropicana had a small adult pool, way off the main pool, at appeared to be open at all hours.