I have been to PR, I usually went to Jobos Beach. From my thoughts.. It isn't good idea to set a camp and sleep on the beach... But I do know one spot you could do that. Keep on road heading east from Jobos Beach, til you will see small spot with rock surrounds and there is ocean, always have good waves on big days. It called Secret's Spot. Often campers slept there since it is in nowhere off middle of the road. You know that hotels are that cheap in Isabella, a big town up the mountain from Jobos beach.
If you want to go other beach than Jobos Beach then I got no idea. You could try Surfer's Beach near Aguadilla airport. You know hotels aren't that expensive in PR during winter.