There are no jet skis or motorized beach sports in Sosua / Cababerete beach/bay to non residents as far as i am aware . There are lots of wind surfers and kite boarders around so motorized sports woud be dangerous,i believe they stopped being used around 2007-08
If you are a resident its not a problem though
However ... there is a guy renting them at La Boca, a river mouth at 5 mins from Cabarete heading east, the only place where you can use motor. You may have to arrange it first in Cabarete tough,sorry i dont have a name for the fella but if your Spanish is good ask around Cabarete and they should be able to have his mobile number but of course don't forget that if you get caught ( in the above two areas ) there could be trouble .