Hi,
A lot of pools will be closed over the Winter season in Las Vegas as it gets cold ( and windy )and some hotels only have a smaller pool open ( ie Mandalay bay only had a small pool open as the beach area was closed for the season). Even the hotels with indoor pools ie Embassy suites are small pools)
There is a wetn'wild in Las Vegas ( South Fort Apache drive ) 20mins from the strip but it is only open Apr-Sep
https://wetnwildlasvegas.com/
Regards
TBS
PS there is a waterpark in Phoenix by the Arizona Grand resort but only reopens in February
https://www.arizonagrandresort.com/oasis-water-park/
Some of the timeshare resorts may have large pools ( There is Tahiti village south of the strip and the webpage has a large pool but unsure of the winter hours)