Hi Annie,
On one trip to St. Maarten, we took a cab to Orient beach where they do rent jet ski's, sorry don't recall the cost. There was also parasailing as well.
I don't know the cost, but you can most likely rent right by the port there in St. Maarten. There is a place--can't think of the name right off the bat, but if you go to Front Street, turn left you will find it.
Try Mullet Bay for jet skiing..It's been a few years but it was a great place to jet ski..Don't know the cost but can plan on $50-60 an hour..that' s being conservative..Have Fun!!