To register your boat, you must visit a local DRNA facility. First, you will have to pay the fees based on the lenght of the boat and obtain a "stamp" at a government collection office. Then, take the stamp, along with your boat information (coast guard registration, if applicable, proof of ownerships, etc) to the DRNA office and they will provide an ID card for your wallet and a sticker that should be installed on your boat.
Here is the link:
The phone number is: 1-787-999-2200
Let me know if you need additional information.