We did last August. Rented a jetski through Elvis (had heard his name on the MB). Asked for him near the CSA watersports area. It didn't take much asking and he found us.
We paid $40 for 1/2 hour and it was long enough for us...plenty of time for the two of us to tour up and down the entire beach..down past Sandals and back and then down to past the town of Negril and back. We rented the jetski to sight see more than zoom around, and enjoyed it a lot.
They don't go with you. My wife and I have done it on both our previous trips and they just tell you where is safe, where is not,and when to be back. GREAT TIME AWSOME PICTURES. Don't forget your camera!