Room reservations and day experience reservations (admission to Aquaventure Waterpark and Lost Chambers Aquarium) can now be made on the Atlantis Sanya website. The first day of availability is May 1, 2018.
Sources: https://www.atlantissanya.com/reservation/day-activities/book-now and http://travel.people.com.cn/n1/2017/1110/c41570-29639021.html