We rented from PortCars. We didn't have any problems at all. When we received our confirmation, they included a pack of coupons and a little beach bag. We used both of them.
We did not visit St Marteen, but I have been to St Thomas & St John before (not with the Disney cruise) and we rented a Jeep for the day on St Johns. The island is not that big, and we saw some beautiful scenery. I would definitely do it again. Just keep in mind (at least in St Thomas & St John) they drive on the other side of the road (the car does has the steering wheel on the Left side of the car).
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