xanadu island resort

Can we go to xanadu island resort when RCI is at Belize? If so what is the best way to get there?

You need to ask the resort if they have day passes. I didn't see any on their website. But if so, yes, you can get there from Belize City via a ferry or by air. If, however, your RCL ship is docking at Harvest Cay, that'll be too far to travel because harvest Cay is further south. Not sure when Harvest Cay will open, or if it is already accepting RCL ships. Your itinerary should be able to state which port in Belize you'll be at.