Happy to help.
We'll typically rent a small pickup truck rather than a car for dive trips such as this. On our first trip to Curacao, we'd reserved a pickup truck but something happened in the system and it wasn't there when we showed up, so we got a Rav4 instead. After several days of trucking around moist dive gear in the heat, it got fairly smelly inside. After some return trips to Curacao and Bonaire, we came to the conclusion a truck is best for segregating the moist dive gear from the cab.
Budget, Centerline, and Hertz all have pickup trucks in their rental fleets on St. Croix. We rented a Nissan Frontier extended cab from Budget for $299 for the week plus takes & fees, came out to about $337 total for the week on our most recent trip in March '09. It was just right for 2 divers, and on the 2 days we had a third passenger, my wife was just petite enough to fit in one of the tiny second row seats. If you'll have more than 2 people, consider a full size truck or a crew cab (either small or large).