They all **can be** heated, but you never know which ones will be warm on any given day. Splashers can vary significantly day to day. I've been there when it was a warm bathwater one day and ice water the next. I imagine that pool has to completely flush itself on occasion. Same thing with the river pool and lazy river by the beach tower. One time it was so warm it was not at all refreshing.
I've been to Atlantis in late February and early March several times. Sometimes it has been too cool and breezy to swim for a day or two, but the weather usually changes and gets comfortable enough to enjoy the water.