Don't get a car. Ours broke down 3 times the last time we visited St. Maarten, so this time we just walked to the beaches and shopping and had a great day! We got chairs on the beach and 6 beers for $20. Every deal on St Maarten includes a beer.
We went to Coki Beach and the water is crystal clear. You can snorkel right of the beach. You have to take a taxi to get there, which was $9 one-way. We bargained with our driver and made it $7, but then a police officer pulled her over (other drivers overheard and called the police because they wanted our business). Anyway, we just hopped out and said we were paying the $9, even though we paid $7 and everything was fine. She didn't care how much we paid because we got her out of trouble.