WE did the jetski tour on Labadee, and you have to sign up for the excursion, they didn't let you just rent them by the hour, but it was still a ton of fun. Yes, during the ride you can switch riders as long as she's old enough to drive. Even though you have to follow the guy in front of you, if you want to go faster, they will let you go in front, they have a lead guy and one that goes last, so they pretty much make sure you don't fool around and get reckless, if you do, you're out, but we were in front and they did let us go very fast, that was not
a problem. It was great, both me and my husband shared driving and it was nice to see other parts of Haiti that way. They also make you watch a movie and get some instruction before they take you out.
Just be sure to sign up early, or even pre-book it on the RCCL web-site if you are 60 days before the cruise, they did have them going all day long, in fact my husband signed up for a 2nd one later in the day.
This link will get you to where you can go in and look/purchase excursions, but you will need your reservation number, which your travel agent should have, but you most likely already have it, have fun: