Not sure about ATV's, but I do know you can rent waverunners at almost all of the public beaches. For sure you can find them at Orient, Greatbay, Divi, Pelican and Mullet Beaches. Hope this helps.
We rented them right at Orient Beach (I believe it was at Kontiki Watersport). We took their Waverunner Tour which was pretty neat - got to snorkel - visit some secluded beaches/bays. If memory serves me correctly the cost for 2 of us was $140 for the tour. My daughter just rented a waverunner for 1/2 hr. and it was $35-$40.