From what I've been able to find, unless you book your air and hotel with
DCL, the transfers are just from the airport to the port.
However, there are many hotels in the Ramblas area that are not far from the port--depending on who I believe, a 5-15 minute cab ride for about $20. No personal experience with this--I'm trying to put it all together for myself! One guy said the port was a 20 minute walk from his hotel in this area. There is one hotel in the Marina (port) area, but it's very expensive.