It looks to me like there are two waterparks. The first one (Barcy) looks included but the second (Pirates Island) is extra charge on a daily basis.
From the hotel website at http://www.barcelo.com/BarceloHotels/en_GB/hotels/dominican-republic/punta-cana-beach-bavaro/hotel-barcelo-bavaro-palace-deluxe/facilities-services.aspx
There is also the Barcy Water Park where children can enjoy the castles, slides and waterfalls inside the pool.
For all the family to enjoy, the new Pirates Island Water Park!! Swimming pool with waves and slides for the more adventurous ones of the family to enjoy. Feel like a brave pirate, slipping and sliding down the chutes until you land with a splash in the challenging waves at this authentic aquatic adventure beneath the hot Caribbean sun. Subject to an additional charge
Additional advice for Pirates Island:Duration: The supplement for admission is for use between 09.00 and 18.00.Additional information:- To book additional days you need to contact the water park’s customer services department directly. Cost per person per day…
– Minimum height of children, 1.2 metres.