If you are an American Express cardholder, they used to offer a parcel holding service
for members abroad, it is worth asking them. But since PR is a US property served by the USPS, you should be able to send things (by mail only) to a main post office addressed to "General Delivery" which the rest of the world knows as "Post Restante" and it will beheld for pickup. Not every postal station offers this--usually just the one or two main post offices in any town so you need to check. That should work
regardless of what postal system (country) you are mailing them from, but other services like UPS or Fedex are not allowed to deliver there. Both of those also offer "hold at depot" where they will hold parcels at their own depots for pickup, but typically they will hold for 3-5 days versus a month for General Delivery
If you are going to be at a marina or other facility, ask them if they'll hold parcels for you. Beware, some hotels will say "Yes!" and forget to tell you they surcharge $10 a piece for holding parcels.