There are two rental car agencies near the dock at St Maarten. They're located in the Great Bay Marina (<a href="http://www.dcltribute.com/images/philipsburg.jpg">map/photo</a>). The closest is an Avis. But the rates they quote on their web site seem rather high to me. I think you could do better by negotiating onsite. I rented from Sunshine Car Rental which had a banner up with the "Dollar" logo. This location doesn't appear on Dollar's web site, though. I paid $35 for a late-model bread box with standard shift (<a href="http://www.dcltribute.com/gallery/p133.htm">photo</a>). But this was during the off-season. I've also heard that there is a Thrifty near the Visitor's Center at the end of the dock, but I don't have first-hand confirmation of this.
At St Thomas, I noticed a Budget location within a 5-minute walk of the ship (<a href="http://www.budgetstt.com/html/havensight.html">link</a>). I've heard that there is an Avis location nearby as well, but, again, I don't have first-hand confirmation of this.
Beware at St Thomas that they drive on the left side of the road (British-style). But the steering wheel is on the left side of the car (American-style), so you're driving near the curb rather than near the center-line of the road.