There is NO ferry from Miami to Puerto Rico. There IS a ferry from Puerto Rico to the Dominican Republic. Your car must be paid for and the licence in your name in order for it to be permitted on the ferry. If your car is not yet paid for, you must get a permit from the bank or company to whom you are paying it allowing you to take it out of
PR. You must buy insurance in case you have an accident in DR. Before boarding in
PR, your car goes through a strict inspection for drugs, guns, and anything else illegal; upon returning to
PR they do the same in DR. When you return to
PR, your car must return with you. I do not know about motorcycles being permitted on the ferry.