I second sail445's recommendation about doing a one-way car rental. We have found it to be less expensive than a taxi and more flexible for provisioning - though the taxi drivers will stop and wait while you provision. I believe we rented through Avis in the past. They have a base in Cieba just a couple miles from the marina and they will give you a lift to the marina after you return the car. The drive from the airport is very easy.

We have provisioned at Ralphs before and had no complaints. It is on the right side of the highway in Fajardo on your way to the marina. However, on our last trip we went to Costco in San Juan which is barely off the route to the marina from SJU - maybe one mile at the most. It was great for stocking up on beverages as well as pretty much whatever else we needed.