Yes, you make prior arrangements. I had booked with these people about 3 years ago. I got there and was going to drop off the car until I saw the conditions. There were cars parked within inches of each other, out on the street, people had been waiting in lines for hours to get their cars back. I decided that I wasn't going to risk parking my car there. I just bit the bullet and parked at the port. At least I knew it was going to be in a semi-secure area, as secure as it's going to get anywhere. It wasn't out on a street and it wasn't parked so I had to climb out of the window. In their defense, there had been a bad snow storm up north and there were a lot of cars that had been stranded just like the owners were. Therefore, the lot was way over-booked and over-flowing. Also, they did give me my money back since I had prepaid for my parking. I doubt they were usually that bad, but I won't take a chance again. They did offer shuttle service too/from the port and the airport. That particular weekend, the shuttle service was very slow. It probably wasn't a good week to judge their performance. The snow storm, Christmas, 8 ships in port, so there were definitely things against them. However, I didn't want to end a wonderful week in the Caribbean by having my car stolen or wrecked.
It was really expensive parking at the port. But it was very nice to come back from a fantastic week, walk out to my car, and be on the road in less than 30 minutes.