I rented a jeep on St John a couple of years ago. I do not recall seeing cars available, though they may have simply been out on rental at the time. The agency (sorry, don't remember their name any longer) was right at the ferry dock, adjacent to the only service station there. Driving on the island was easy since there are so few roads. The roads are, however, narrow and winding, as well as occasionally occupied by wild goats. So, you really need to keep your speed down. I will tell you that the jeep rental was expensive for an entire day -- perhaps triple what a stateside car rental would be.
I don't know whether you are looking to go to a particular place or taking a general tour. If touring is what you're after, there are some fairly reasonable open air vehicles that seat 15 - 20 people that could accomodate your entire group on a 2-3 hour narrated tour of the island complete with a couple of quick stops.
I have been to St Maarten but never rented a car there. Taxi's are very easy to get and very reliable for either general transportation or touring. There will be lots of them at the cruise dock, as well as the water taxi landing downtown. Do make sure you confirm the price, though, before taking off in the taxi!
Ted