I have never been to Puerto Rico or Culebra and I understand that most people can speak English. My question is, am I going to be able to practice my Spanish there or am I only going to be hearing English all the time? I speak fluent spanish but it's been a few years since I've used it and it would be fun to "brush up" a little while I'm there. What about Old San Juan? Do most people speak English or Spanish there?
You'll have ample opportunity to use Spanish. The native Culebrenses are always appreciative of my less than fluent efforts, and doing so has improved my skills (such as they are) and confidence.