If the water park is in the territory of a hotel, then only guests of this hotel can visit it. The only exception is the water park in the Sirenis Punta Cana hotel - all tourists from all hotels can come to it, but the entrance to it is paid.

If you purchase a special pass (Day use) per day for an additional fee, you can go to the hotel and use its infrastructure, although there will certainly be some restrictions. But, in general, this is possible.