Pretty much all of the major beaches at each port do jet ski rentals. I know for a fact that you can rent them at the Atlantis if you plan on going there. I usually find it more fun doing an actual jet ski excursion though. It'll be cheaper to just rent somewhere, but when you book an excursion you get to go with a group of people and you will be more comfortable going out further in the ocean because you don't have to worry about finding your way back.
Just be careful! Especially if you are not use to the jet skis in the ocean with waves. One of my friends flipped his with a wave and his necklace got caught on the handle bars. Was bad with the cut on his neck and bump on his head. This happened at Belize
We did one in Jamaica (he rode with me HA HA) and got 2 in Labadee. It was a blast but the ride in Labadee was intense and our guide stayed way too far ahead of everyone so it was really hard to keep up. Full throttle with huge waves, I felt like I was riding a jack-hammer!