Arrange a taxi.
Check with Royal Caribbean to see if they provide coach connections. Most cruise companies arrange coach travel to their ships from various departure points around the country. If so, you may be able to pick one of these up from somewhere close, if not from the airport itself. Failing that there must be an independent coach service of some sort between these two places surely? Train? A taxi would be most convenient, but also no doubt the most expensive! Happy cruising.