Whaaw !! I don't come here for 4 days and I have more than 10 pages to catch up...You guys talk to much
Well I will be very happy to help if you got some questions about St Martin !!
But unfortunatly I cannot help so much with car retal compagnies...

Maybe a quick search on Google could help, I'm pretty sure the big compagnies (like "Hertz") are represented
But just a thing about have your "own" car in St Martin : the road are ok and it's very simple to getting oriented, but the traffic on this island can be HUGE!
So allow enough time to get back to the ship ^^
For example if you spend some time at Orient Beach (a very poular place on the north) allow one hour to come back to be sure you won't miss the Magic
A good thing too, if you are sure where you want to go, which place you want to visit, you can share a taxi with anoter couple