This website gives cruise only prices for ocean village 2 and voyager of the seas with sail from palma and barcelona, its best to call and check availability and prices on lastminute cruises.
Try island cruises sail from palma try them.
English newspapers, for example, The Costa Blanca News are always advertising cruise only deals from Barcelona with Royal Caribbean Cruise Line, sometimes with coach travel to and from the port. Otherwise cruise only deals can be bought direct from the Royal Caribbean web site. However these deals do not have the ABTA guarantees that we would have booking the same cruise with a UK travel agent.