It really all depends on what sort of beach you want ..the beaches on the south side, the caribbean, have mainly very flat but clear water and for activities the best are in the Bavaro/Punta Cana area and for beautiful quieter areas in the SW area .
On the noth side , the Atlantic, the beahces have rougher water , can be very dangerous in several beaches such as Playa Grande but the atmosphere is more laid back and Cabrera, for example, is perfect for people in their twenties . There are beaches on the Samana peninsula but I have never found them as atractive as the northern beaches more to the west .